SEARCH
NEW RPMS
DIRECTORIES
ABOUT
FAQ
VARIOUS
BLOG

BotDetect - Real-Time Bot Detection API
 
 

MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLTRAP

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index 

NAME

perltrap - Perl traps for the unwary 

DESCRIPTION

The biggest trap of all is forgetting to use the -w switch; seethe perlrun manpage. The second biggest trap is not making your entire programrunnable under use strict. The third biggest trap is not readingthe list of changes in this version of Perl; see the perldelta manpage. 

Awk Traps

Accustomed awk users should take special note of the following:
*
The English module, loaded via

    use English;
allows you to refer to special variables (like $/) with names (like$RS), as though they were in awk; see the perlvar manpage for details.
*
Semicolons are required after all simple statements in Perl (exceptat the end of a block). Newline is not a statement delimiter.
*
Curly brackets are required on ifs and whiles.
*
Variables begin with ``$'', ``@'' or ``%'' in Perl.
*
Arrays index from 0. Likewise string positions in substr() andindex().
*
You have to decide whether your array has numeric or string indices.
*
Hash values do not spring into existence upon mere reference.
*
You have to decide whether you want to use string or numericcomparisons.
*
Reading an input line does not split it for you. You get to split itto an array yourself. And the split() operator has differentarguments than awk's.
*
The current input line is normally in $_, not $0. It generally doesnot have the newline stripped. ($0 is the name of the programexecuted.) See the perlvar manpage.
*
$<digit> does not refer to fields---it refers to substrings matchedby the last match pattern.
*
The print() statement does not add field and record separators unlessyou set $, and $\. You can set $OFS and $ORS if you're usingthe English module.
*
You must open your files before you print to them.
*
The range operator is ``..'', not comma. The comma operator works as inC.
*
The match operator is ``=~'', not ``~''. ("~'' is the one's complementoperator, as in C.)
*
The exponentiation operator is ``**'', not ``^''. ``^'' is the XORoperator, as in C. (You know, one could get the feeling that awk isbasically incompatible with C.)
*
The concatenation operator is ``.'', not the null string. (Using thenull string would render /pat/ /pat/ unparsable, because the third slashwould be interpreted as a division operator---the tokenizer is in factslightly context sensitive for operators like ``/'', ``?'', and ``>''.And in fact, ``.'' itself can be the beginning of a number.)
*
The next, exit, and continue keywords work differently.
*
The following variables work differently:

      Awk       Perl      ARGC      $#ARGV or scalar @ARGV      ARGV[0]   $0      FILENAME  $ARGV      FNR       $. - something      FS        (whatever you like)      NF        $#Fld, or some such      NR        $.      OFMT      $#      OFS       $,      ORS       $\      RLENGTH   length($&)      RS        $/      RSTART    length($`)      SUBSEP    $;

*
You cannot set $RS to a pattern, only a string.
*
When in doubt, run the awk construct through a2p and see what itgives you.
 

C Traps

Cerebral C programmers should take note of the following:
*
Curly brackets are required on if's and while's.
*
You must use elsif rather than else if.
*
The break and continue keywords from C become inPerl last and next, respectively.Unlike in C, these do NOT work within a do { } while construct.
*
There's no switch statement. (But it's easy to build one on the fly.)
*
Variables begin with ``$'', ``@'' or ``%'' in Perl.
*
printf() does not implement the ``*'' format for interpolatingfield widths, but it's trivial to use interpolation of double-quotedstrings to achieve the same effect.
*
Comments begin with ``#'', not ``/*''.
*
You can't take the address of anything, although a similar operatorin Perl is the backslash, which creates a reference.
*
ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0]ends up in $0.
*
System calls such as link(), unlink(), rename(), etc. return nonzero forsuccess, not 0.
*
Signal handlers deal with signal names, not numbers. Use kill -lto find their names on your system.
 

Sed Traps

Seasoned sed programmers should take note of the following:
*
Backreferences in substitutions use ``$'' rather than ``\''.
*
The pattern matching metacharacters ``('', ``)'', and ``|'' do not have backslashesin front.
*
The range operator is ..., rather than comma.
 

Shell Traps

Sharp shell programmers should take note of the following:
*
The backtick operator does variable interpolation without regard tothe presence of single quotes in the command.
*
The backtick operator does no translation of the return value, unlike csh.
*
Shells (especially csh) do several levels of substitution on eachcommand line. Perl does substitution in only certain constructssuch as double quotes, backticks, angle brackets, and search patterns.
*
Shells interpret scripts a little bit at a time. Perl compiles theentire program before executing it (except for BEGIN blocks, whichexecute at compile time).
*
The arguments are available via @ARGV, not $1, $2, etc.
*
The environment is not automatically made available as separate scalarvariables.
 

Perl Traps

Practicing Perl Programmers should take note of the following:
*
Remember that many operations behave differently in a listcontext than they do in a scalar one. See the perldata manpage for details.
*
Avoid barewords if you can, especially all lowercase ones.You can't tell by just looking at it whether a bareword isa function or a string. By using quotes on strings andparentheses on function calls, you won't ever get them confused.
*
You cannot discern from mere inspection which builtinsare unary operators (like chop() and chdir())and which are list operators (like print() and unlink()).(User-defined subroutines can be only list operators, neverunary ones.) See the perlop manpage.
*
People have a hard time remembering that some functionsdefault to $_, or @ARGV, or whatever, but that others whichyou might expect to do not.
*
The <FH> construct is not the name of the filehandle, it is a readlineoperation on that handle. The data read is assigned to $_ only if thefile read is the sole condition in a while loop:

    while (<FH>)      { }    while (defined($_ = <FH>)) { }..    <FH>;  # data discarded!

*
Remember not to use ``='' when you need ``=~'';these two constructs are quite different:

    $x =  /foo/;    $x =~ /foo/;

*
The do {} construct isn't a real loop that you can useloop control on.
*
Use my() for local variables whenever you can get away withit (but see the perlform manpage for where you can't).Using local() actually gives a local value to a globalvariable, which leaves you open to unforeseen side-effectsof dynamic scoping.
*
If you localize an exported variable in a module, its exported value willnot change. The local name becomes an alias to a new value but theexternal name is still an alias for the original.
 

Perl4 to Perl5 Traps

Practicing Perl4 Programmers should take note of the followingPerl4-to-Perl5 specific traps.

They're crudely ordered according to the following list:

Discontinuance, Deprecation, and BugFix traps
Anything that's been fixed as a perl4 bug, removed as a perl4 featureor deprecated as a perl4 feature with the intent to encourage usage ofsome other perl5 feature.
Parsing Traps
Traps that appear to stem from the new parser.
Numerical Traps
Traps having to do with numerical or mathematical operators.
General data type traps
Traps involving perl standard data types.
Context Traps - scalar, list contexts
Traps related to context within lists, scalar statements/declarations.
Precedence Traps
Traps related to the precedence of parsing, evaluation, and execution ofcode.
General Regular Expression Traps using s///, etc.
Traps related to the use of pattern matching.
Subroutine, Signal, Sorting Traps
Traps related to the use of signals and signal handlers, general subroutines,and sorting, along with sorting subroutines.
OS Traps
OS-specific traps.
DBM Traps
Traps specific to the use of dbmopen(), and specific dbm implementations.
Unclassified Traps
Everything else.

If you find an example of a conversion trap that is not listed here,please submit it to Bill Middleton <wjmAATTbest.com> for inclusion.Also note that at least some of these can be caught with -w. 

Discontinuance, Deprecation, and BugFix traps

Anything that has been discontinued, deprecated, or fixed asa bug from perl4.
* Discontinuance
Symbols starting with ``_'' are no longer forced into package main, exceptfor $_ itself (and @_, etc.).

    package test;    $_legacy = 1;
    package main;    print "\$_legacy is ",$_legacy,"\n";
    # perl4 prints: $_legacy is 1    # perl5 prints: $_legacy is

* Deprecation
Double-colon is now a valid package separator in a variable name. Thus thesebehave differently in perl4 vs. perl5, because the packages don't exist.

    $a=1;$b=2;$c=3;$var=4;    print "$a::$b::$c ";    print "$var::abc::xyz\n";     # perl4 prints: 1::2::3 4::abc::xyz    # perl5 prints: 3
Given that :: is now the preferred package delimiter, it is debatablewhether this should be classed as a bug or not.(The older package delimiter, ` ,is used here)

    $x = 10 ;    print "x=${'x}\n" ;
    # perl4 prints: x=10    # perl5 prints: Can't find string terminator "'" anywhere before EOF
You can avoid this problem, and remain compatible with perl4, if youalways explicitly include the package name:

    $x = 10 ;    print "x=${main'x}\n" ;
Also see precedence traps, for parsing $:.
* BugFix
The second and third arguments of splice() are now evaluated in scalarcontext (as the Camel says) rather than list context.

    sub sub1{return(0,2) }          # return a 2-elem array    sub sub2{ return(1,2,3)}        # return a 3-elem array    @a1 = ("a","b","c","d","e");    @a2 = splice(@a1,&sub1,&sub2);    print join(' ',@a2),"\n";
    # perl4 prints: a b    # perl5 prints: c d e

* Discontinuance
You can't do a goto into a block that is optimized away. Darn.

    goto marker1;
    for(1){    marker1:        print "Here I is!\n";    }
    # perl4 prints: Here I is!    # perl5 dumps core (SEGV)

* Discontinuance
It is no longer syntactically legal to use whitespace as the nameof a variable, or as a delimiter for any kind of quote construct.Double darn.

    $a = ("foo bar");    $b = q baz ;    print "a is $a, b is $b\n";
    # perl4 prints: a is foo bar, b is baz    # perl5 errors: Bareword found where operator expected

* Discontinuance
The archaic while/if BLOCK BLOCK syntax is no longer supported.

    if { 1 } {        print "True!";    }    else {        print "False!";    }
    # perl4 prints: True!    # perl5 errors: syntax error at test.pl line 1, near "if {"

* BugFix
The ** operator now binds more tightly than unary minus.It was documented to work this way before, but didn't.

    print -4**2,"\n";
    # perl4 prints: 16    # perl5 prints: -16

* Discontinuance
The meaning of foreach{} has changed slightly when it is iterating over alist which is not an array. This used to assign the list to atemporary array, but no longer does so (for efficiency). This meansthat you'll now be iterating over the actual values, not over copies ofthe values. Modifications to the loop variable can change the originalvalues.

    @list = ('ab','abc','bcd','def');    foreach $var (grep(/ab/,@list)){        $var = 1;    }    print (join(':',@list));
    # perl4 prints: ab:abc:bcd:def    # perl5 prints: 1:1:bcd:def
To retain Perl4 semantics you need to assign your listexplicitly to a temporary array and then iterate over that. Forexample, you might need to change

    foreach $var (grep(/ab/,@list)){
to

    foreach $var (@tmp = grep(/ab/,@list)){
Otherwise changing $var will clobber the values of @list. (This most oftenhappens when you use $_ for the loop variable, and call subroutines inthe loop that don't properly localize $_.)
* Discontinuance
split with no arguments now behaves like split ' ' (which doesn'treturn an initial null field if $_ starts with whitespace), it used tobehave like split /\s+/ (which does).

    $_ = ' hi mom';    print join(':', split);
    # perl4 prints: :hi:mom    # perl5 prints: hi:mom

* BugFix
Perl 4 would ignore any text which was attached to an -e switch,always taking the code snippet from the following arg. Additionally, itwould silently accept an -e switch without a following arg. Both ofthese behaviors have been fixed.

    perl -e'print "attached to -e"' 'print "separate arg"'
    # perl4 prints: separate arg    # perl5 prints: attached to -e
    perl -e
    # perl4 prints:    # perl5 dies: No code specified for -e.

* Discontinuance
In Perl 4 the return value of push was undocumented, but it wasactually the last value being pushed onto the target list. In Perl 5the return value of push is documented, but has changed, it is thenumber of elements in the resulting list.

    @x = ('existing');    print push(@x, 'first new', 'second new');
    # perl4 prints: second new    # perl5 prints: 3

* Discontinuance
In Perl 4 (and versions of Perl 5 before 5.004), '\r' characters inPerl code were silently allowed, although they could cause (mysterious!)failures in certain constructs, particularly here documents. Now,'\r' characters cause an immediate fatal error. (Note: In thisexample, the notation \015 represents the incorrect lineending. Depending upon your text viewer, it will look different.)

    print "foo";\015    print "bar";
    # perl4     prints: foobar    # perl5.003 prints: foobar    # perl5.004 dies: Illegal character \015 (carriage return)
See the perldiag manpage for full details.
* Deprecation
Some error messages will be different.
* Discontinuance
Some bugs may have been inadvertently removed. :-)
 

Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.
* Parsing
Note the space between . and =

    $string . = "more string";    print $string;
    # perl4 prints: more string    # perl5 prints: syntax error at - line 1, near ". ="

* Parsing
Better parsing in perl 5

    sub foo {}    &foo    print("hello, world\n");
    # perl4 prints: hello, world    # perl5 prints: syntax error

* Parsing
``if it looks like a function, it is a function'' rule.

  print    ($foo == 1) ? "is one\n" : "is zero\n";
    # perl4 prints: is zero    # perl5 warns: "Useless use of a constant in void context" if using -w
 

Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators,operands, or output from same.
* Numerical
Formatted output and significant digits

    print 7.373504 - 0, "\n";    printf "%20.18f\n", 7.373504 - 0;
    # Perl4 prints:    7.375039999999996141    7.37503999999999614
    # Perl5 prints:    7.373504    7.37503999999999614

* Numerical
This specific item has been deleted. It demonstrated how the auto-incrementoperator would not catch when a number went over the signed int limit. Fixedin version 5.003_04. But always be wary when using large integers.If in doubt:

   use Math::BigInt;

* Numerical
Assignment of return values from numeric equality testsdoes not work in perl5 when the test evaluates to false (0).Logical tests now return an null, instead of 0

    $p = ($test == 1);    print $p,"\n";
    # perl4 prints: 0    # perl5 prints:
Also see the section on General Regular Expression Traps using s///, etc.for another example of this new feature...
 

General data type traps

Perl4-to-Perl5 traps involving most data-types, and their usagewithin certain expressions and/or context.
* (Arrays)
Negative array subscripts now count from the end of the array.

    @a = (1, 2, 3, 4, 5);    print "The third element of the array is $a[3] also expressed as $a[-2] \n";
    # perl4 prints: The third element of the array is 4 also expressed as    # perl5 prints: The third element of the array is 4 also expressed as 4

* (Arrays)
Setting $#array lower now discards array elements, and makes themimpossible to recover.

    @a = (a,b,c,d,e);    print "Before: ",join('',@a);    $#a =1;    print ", After: ",join('',@a);    $#a =3;    print ", Recovered: ",join('',@a),"\n";
    # perl4 prints: Before: abcde, After: ab, Recovered: abcd    # perl5 prints: Before: abcde, After: ab, Recovered: ab

* (Hashes)
Hashes get defined before use

    local($s,@a,%h);    die "scalar \$s defined" if defined($s);    die "array \@a defined" if defined(@a);    die "hash \%h defined" if defined(%h);
    # perl4 prints:    # perl5 dies: hash %h defined

* (Globs)
glob assignment from variable to variable will fail if the assignedvariable is localized subsequent to the assignment

    @a = ("This is Perl 4");    *b = *a;    local(@a);    print @b,"\n";
    # perl4 prints: This is Perl 4    # perl5 prints:
    # Another example
    *fred = *barney; # fred is aliased to barney    @barney = (1, 2, 4);    # @fred;    print "@fred";  # should print "1, 2, 4"
    # perl4 prints: 1 2 4    # perl5 prints: In string, @fred now must be written as \@fred

* (Scalar String)
Changes in unary negation (of strings)This change effects both the return value and what itdoes to auto(magic)increment.

    $x = "aaa";    print ++$x," : ";    print -$x," : ";    print ++$x,"\n";
    # perl4 prints: aab : -0 : 1    # perl5 prints: aab : -aab : aac

* (Constants)
perl 4 lets you modify constants:

    $foo = "x";    &mod($foo);    for ($x = 0; $x < 3; $x++) {        &mod("a");    }    sub mod {        print "before: $_[0]";        $_[0] = "m";        print "  after: $_[0]\n";    }
    # perl4:    # before: x  after: m    # before: a  after: m    # before: m  after: m    # before: m  after: m
    # Perl5:    # before: x  after: m    # Modification of a read-only value attempted at foo.pl line 12.    # before: a

* (Scalars)
The behavior is slightly different for:

    print "$x", defined $x
    # perl 4: 1    # perl 5: <no output, $x is not called into existence>

* (Variable Suicide)
Variable suicide behavior is more consistent under Perl 5.Perl5 exhibits the same behavior for hashes and scalars,that perl4 exhibits for only scalars.

    $aGlobal{ "aKey" } = "global value";    print "MAIN:", $aGlobal{"aKey"}, "\n";    $GlobalLevel = 0;    &test( *aGlobal );
    sub test {        local( *theArgument ) = @_;        local( %aNewLocal ); # perl 4 != 5.001l,m        $aNewLocal{"aKey"} = "this should never appear";        print "SUB: ", $theArgument{"aKey"}, "\n";        $aNewLocal{"aKey"} = "level $GlobalLevel";   # what should print        $GlobalLevel++;        if( $GlobalLevel<4 ) {            &test( *aNewLocal );        }    }
    # Perl4:    # MAIN:global value    # SUB: global value    # SUB: level 0    # SUB: level 1    # SUB: level 2
    # Perl5:    # MAIN:global value    # SUB: global value    # SUB: this should never appear    # SUB: this should never appear    # SUB: this should never appear
 

Context Traps - scalar, list contexts


* (list context)
The elements of argument lists for formats are now evaluated in listcontext. This means you can interpolate list values now.

    @fmt = ("foo","bar","baz");    format STDOUT=    @<<<<< @||||| @>>>>>    @fmt;    .    write;
    # perl4 errors:  Please use commas to separate fields in file    # perl5 prints: foo     bar      baz

* (scalar context)
The caller() function now returns a false value in a scalar contextif there is no caller. This lets library files determine if they'rebeing required.

    caller() ? (print "You rang?\n") : (print "Got a 0\n");
    # perl4 errors: There is no caller    # perl5 prints: Got a 0

* (scalar context)
The comma operator in a scalar context is now guaranteed to give ascalar context to its arguments.

    @y= ('a','b','c');    $x = (1, 2, @y);    print "x = $x\n";
    # Perl4 prints:  x = c   # Thinks list context interpolates list    # Perl5 prints:  x = 3   # Knows scalar uses length of list

* (list, builtin)
sprintf() funkiness (array argument converted to scalar array count)This test could be added to t/op/sprintf.t

    @z = ('%s%s', 'foo', 'bar');    $x = sprintf(@z);    if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
    # perl4 prints: ok 2    # perl5 prints: not ok 2
printf() works fine, though:

    printf STDOUT (@z);    print "\n";
    # perl4 prints: foobar    # perl5 prints: foobar
Probably a bug.
 

Precedence Traps

Perl4-to-Perl5 traps involving precedence order.
* Precedence
LHS vs. RHS of any assignment operator. LHS is evaluated firstin perl4, second in perl5; this can affect the relationshipbetween side-effects in sub-expressions.

    @arr = ( 'left', 'right' );    $a{shift @arr} = shift @arr;    print join( ' ', keys %a );
    # perl4 prints: left    # perl5 prints: right

* Precedence
These are now semantic errors because of precedence:

    @list = (1,2,3,4,5);    %map = ("a",1,"b",2,"c",3,"d",4);    $n = shift @list + 2;   # first item in list plus 2    print "n is $n, ";    $m = keys %map + 2;     # number of items in hash plus 2    print "m is $m\n";
    # perl4 prints: n is 3, m is 6    # perl5 errors and fails to compile

* Precedence
The precedence of assignment operators is now the same as the precedenceof assignment. Perl 4 mistakenly gave them the precedence of the associatedoperator. So you now must parenthesize them in expressions like

    /foo/ ? ($a += 2) : ($a -= 2);
Otherwise

    /foo/ ? $a += 2 : $a -= 2
would be erroneously parsed as

    (/foo/ ? $a += 2 : $a) -= 2;
On the other hand,

    $a += /foo/ ? 1 : 2;
now works as a C programmer would expect.
* Precedence

    open FOO || die;
is now incorrect. You need parentheses around the filehandle.Otherwise, perl5 leaves the statement as its default precedence:

    open(FOO || die);
    # perl4 opens or dies    # perl5 errors: Precedence problem: open FOO should be open(FOO)

* Precedence
perl4 gives the special variable, $: precedence, where perl5treats $:: as main package

    $a = "x"; print "$::a";
    # perl 4 prints: -:a    # perl 5 prints: x

* Precedence
concatenation precedence over filetest operator?

    -e $foo .= "q"
    # perl4 prints: no output    # perl5 prints: Can't modify -e in concatenation
 

General Regular Expression Traps using s///, etc.

All types of RE traps.
* Regular Expression
s'$lhs'$rhs' now does no interpolation on either side. It used tointerpolate $lhs but not $rhs. (And still does not match a literal'$' in string)

    $a=1;$b=2;    $string = '1 2 $a $b';    $string =~ s'$a'$b';    print $string,"\n";
    # perl4 prints: $b 2 $a $b    # perl5 prints: 1 2 $a $b

* Regular Expression
m//g now attaches its state to the searched string rather than theregular expression. (Once the scope of a block is left for the sub, thestate of the searched string is lost)

    $_ = "ababab";    while(m/ab/g){        &doit("blah");    }    sub doit{local($_) = shift; print "Got $_ "}
    # perl4 prints: blah blah blah    # perl5 prints: infinite loop blah...

* Regular Expression
Currently, if you use the m//o qualifier on a regular expressionwithin an anonymous sub, all closures generated from that anonymoussub will use the regular expression as it was compiled when it was usedthe very first time in any such closure. For instance, if you say

    sub build_match {        my($left,$right) = @_;        return sub { $_[0] =~ /$left stuff $right/o; };    }
build_match() will always return a sub which matches the contents of$left and $right as they were the first time that build_match()was called, not as they are in the current call.

This is probably a bug, and may change in future versions of Perl.

* Regular Expression
If no parentheses are used in a match, Perl4 sets $+ tothe whole match, just like $&. Perl5 does not.

    "abcdef" =~ /b.*e/;    print "\$+ = $+\n";
    # perl4 prints: bcde    # perl5 prints:

* Regular Expression
substitution now returns the null string if it fails

    $string = "test";    $value = ($string =~ s/foo//);    print $value, "\n";
    # perl4 prints: 0    # perl5 prints:
Also see the section on Numerical Traps for another example of this new feature.
* Regular Expression
s`lhs`rhs` (using backticks) is now a normal substitution, with nobacktick expansion

    $string = "";    $string =~ s`^`hostname`;    print $string, "\n";
    # perl4 prints: <the local hostname>    # perl5 prints: hostname

* Regular Expression
Stricter parsing of variables used in regular expressions

    s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
    # perl4: compiles w/o error    # perl5: with Scalar found where operator expected ..., near "$opt$plus"
an added component of this example, apparently from the same script, isthe actual value of the s'd string after the substitution.[$opt] is a character class in perl4 and an array subscript in perl5

    $grpc = 'a';    $opt  = 'r';    $_ = 'bar';    s/^([^$grpc]*$grpc[$opt]?)/foo/;    print ;
    # perl4 prints: foo    # perl5 prints: foobar

* Regular Expression
Under perl5, m?x? matches only once, like ?x?. Under perl4, it matchedrepeatedly, like /x/ or m!x!.

    $test = "once";    sub match { $test =~ m?once?; }    &match();    if( &match() ) {        # m?x? matches more then once        print "perl4\n";    } else {        # m?x? matches only once        print "perl5\n";    }
    # perl4 prints: perl4    # perl5 prints: perl5

* Regular Expression
Under perl4 and upto version 5.003, a failed m//g match used toreset the internal iterator, so that subsequent m//g match attemptsbegan from the beginning of the string. In perl version 5.004 and later,failed m//g matches do not reset the iterator position (which can befound using the pos() function---see the pos entry in the perlfunc manpage).

    $test = "foop";    for (1..3) {        print $1 while ($test =~ /(o)/g);        # pos $test = 0;     # to get old behavior    }
    # perl4     prints: oooooo    # perl5.004 prints: oo
You may always reset the iterator yourself as shown in the commented lineto get the old behavior.
 

Subroutine, Signal, Sorting Traps

The general group of Perl4-to-Perl5 traps having to do withSignals, Sorting, and their related subroutines, as well asgeneral subroutine traps. Includes some OS-Specific traps.
* (Signals)
Barewords that used to look like strings to Perl will now look like subroutinecalls if a subroutine by that name is defined before the compiler sees them.

    sub SeeYa { warn"Hasta la vista, baby!" }    $SIG{'TERM'} = SeeYa;    print "SIGTERM is now $SIG{'TERM'}\n";
    # perl4 prints: SIGTERM is main'SeeYa    # perl5 prints: SIGTERM is now main::1
Use -w to catch this one
* (Sort Subroutine)
reverse is no longer allowed as the name of a sort subroutine.

    sub reverse{ print "yup "; $a <=> $b }    print sort reverse a,b,c;
    # perl4 prints: yup yup yup yup abc    # perl5 prints: abc

* warn() won't let you specify a filehandle.
Although it _always_ printed to STDERR, warn() would let you specify afilehandle in perl4. With perl5 it does not.

    warn STDERR "Foo!";
    # perl4 prints: Foo!    # perl5 prints: String found where operator expected
 

OS Traps


* (SysV)
Under HPUX, and some other SysV OSes, one had to reset any signal handler,within the signal handler function, each time a signal was handled withperl4. With perl5, the reset is now done correctly. Any code relyingon the handler _not_ being reset will have to be reworked.

Since version 5.002, Perl uses sigaction() under SysV.

    sub gotit {        print "Got @_... ";    }    $SIG{'INT'} = 'gotit';
    $| = 1;    $pid = fork;    if ($pid) {        kill('INT', $pid);        sleep(1);        kill('INT', $pid);    } else {        while (1) {sleep(10);}    }
    # perl4 (HPUX) prints: Got INT...    # perl5 (HPUX) prints: Got INT... Got INT...

* (SysV)
Under SysV OSes, seek() on a file opened to append >> now doesthe right thing w.r.t. the fopen() manpage. e.g., - When a file is openedfor append, it is impossible to overwrite information already inthe file.

    open(TEST,">>seek.test");    $start = tell TEST ;    foreach(1 .. 9){        print TEST "$_ ";    }    $end = tell TEST ;    seek(TEST,$start,0);    print TEST "18 characters here";
    # perl4 (solaris) seek.test has: 18 characters here    # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
 

Interpolation Traps

Perl4-to-Perl5 traps having to do with how things get interpolatedwithin certain expressions, statements, contexts, or whatever.
* Interpolation
@ now always interpolates an array in double-quotish strings.

    print "To: someoneAATTsomewhere.com\n";
    # perl4 prints: To:someoneAATTsomewhere.com    # perl5 errors : In string, @somewhere now must be written as \@somewhere

* Interpolation
Double-quoted strings may no longer end with an unescaped $ or @.

    $foo = "foo$";    $bar = "bar@";    print "foo is $foo, bar is $bar\n";
    # perl4 prints: foo is foo$, bar is bar@    # perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
* Interpolation
Perl now sometimes evaluates arbitrary expressions inside braces that occurwithin double quotes (usually when the opening brace is preceded by $or @).

    @www = "buz";    $foo = "foo";    $bar = "bar";    sub foo { return "bar" };    print "|@{w.w.w}|${main'foo}|";
    # perl4 prints: |@{w.w.w}|foo|    # perl5 prints: |buz|bar|
Note that you can use strict; to ward off such trappiness under perl5.
* Interpolation
The construct ``this is $$x'' used to interpolate the pid at thatpoint, but now apparently tries to dereference $x. $$ by itself stillworks fine, however.

    print "this is $$x\n";
    # perl4 prints: this is XXXx   (XXX is the current pid)    # perl5 prints: this is

* Interpolation
Creation of hashes on the fly with eval "EXPR" now requires either both$'s to be protected in the specification of the hash name, or both curliesto be protected. If both curlies are protected, the result will be compatiblewith perl4 and perl5. This is a very common practice, and should be changedto use the block form of eval{} if possible.

    $hashname = "foobar";    $key = "baz";    $value = 1234;    eval "\$$hashname{'$key'} = q|$value|";    (defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");
    # perl4 prints: Yup    # perl5 prints: Nope
Changing

    eval "\$$hashname{'$key'} = q|$value|";
to

    eval "\$\$hashname{'$key'} = q|$value|";
causes the following result:

    # perl4 prints: Nope    # perl5 prints: Yup
or, changing to

    eval "\$$hashname\{'$key'\} = q|$value|";
causes the following result:

    # perl4 prints: Yup    # perl5 prints: Yup    # and is compatible for both versions

* Interpolation
perl4 programs which unconsciously rely on the bugs in earlier perl versions.

    perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
    # perl4 prints: This is not perl5    # perl5 prints: This is perl5

* Interpolation
You also have to be careful about array references.

    print "$foo{"
    perl 4 prints: {    perl 5 prints: syntax error

* Interpolation
Similarly, watch out for:

    $foo = "array";    print "\$$foo{bar}\n";
    # perl4 prints: $array{bar}    # perl5 prints: $
Perl 5 is looking for $array{bar} which doesn't exist, but perl 4 ishappy just to expand $foo to ``array'' by itself. Watch out for thisespecially in eval's.
* Interpolation
qq() string passed to eval

    eval qq(        foreach \$y (keys %\$x\) {            \$count++;        }    );
    # perl4 runs this ok    # perl5 prints: Can't find string terminator ")"
 

DBM Traps

General DBM traps.
* DBM
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)may cause the same script, run under perl5, to fail. The build of perl5must have been linked with the same dbm/ndbm as the default for dbmopen()to function properly without tie'ing to an extension dbm implementation.

    dbmopen (%dbm, "file", undef);    print "ok\n";
    # perl4 prints: ok    # perl5 prints: ok (IFF linked with -ldbm or -lndbm)

* DBM
Existing dbm databases created under perl4 (or any other dbm/ndbm tool)may cause the same script, run under perl5, to fail. The error generatedwhen exceeding the limit on the key/value size will cause perl5 to exitimmediately.

    dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";    $DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm    print "YUP\n";
    # perl4 prints:    dbm store returned -1, errno 28, key "trap" at - line 3.    YUP
    # perl5 prints:    dbm store returned -1, errno 28, key "trap" at - line 3.
 

Unclassified Traps

Everything else.
* require/do trap using returned value
If the file doit.pl has:

    sub foo {        $rc = do "./do.pl";        return 8;    }    print &foo, "\n";
And the do.pl file has the following single line:

    return 3;
Running doit.pl gives the following:

    # perl 4 prints: 3 (aborts the subroutine early)    # perl 5 prints: 8
Same behavior if you replace do with require.
* split on empty string with LIMIT specified

        $string = '';    @list = split(/foo/, $string, 2)
Perl4 returns a one element list containing the empty string but Perl5returns an empty list.

As always, if any of these are ever officially declared as bugs,they'll be fixed and removed.

 

Index

NAME
DESCRIPTION
Awk Traps
C Traps
Sed Traps
Shell Traps
Perl Traps
Perl4 to Perl5 Traps
Discontinuance, Deprecation, and BugFix traps
Parsing Traps
Numerical Traps
General data type traps
Context Traps - scalar, list contexts
Precedence Traps
General Regular Expression Traps using s///, etc.
Subroutine, Signal, Sorting Traps
OS Traps
Interpolation Traps
DBM Traps
Unclassified Traps
This document was created byman2html,using the manual pages.
  
ICM Bot detect detector