I just spent two days finding a bug in a complex program, which turned out to be related to $_... again (see http://www.perlmonks.org/?node_id=1154900). Here is a short version of the problem I had:

#!/usr/bin/perl

use strict;
use warnings;

use bug::myitem;

sub generate {
    my $list = shift;
    my @data;

    my $i=0;
    foreach my $f (@$list) {
    my $item = bug::myitem->new($i++, $f);
    push(@data, [$item, $i]);
    }
    return \@data;
}


my $data;
if (scalar(@ARGV)>0) {
    $data = generate(\@ARGV);
} else {
    die "syntax: <file1> <file2> ..."
}

print "Before:\n";
foreach my $item (@$data) {
    print "item->[0]=".$item->[0]." ; item->[1]=".$item->[1]."\n";
}

my $dataStuff = map { $_->[0]->getContent() } @$data;

print "After:\n";
foreach my $item (@$data) {
    print "item->[0]=".$item->[0]." ; item->[1]=".$item->[1]."\n";
}
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and the class is this:

package bug::myitem;

use strict;
use warnings;

sub new {
    my ($class, $id, $file) = @_;

    my $self;
    $self->{id} = $id;
    $self->{file} = $file;
    bless($self, $class);
    return $self;
}


sub getContent() {
    my $self = shift;
    my $fh;
    my $res;
    open($fh, "<",$self->{file}) or die "cannot open '".$self->{file}.
+"'";
    while (<$fh>) {
    chomp;
    $res .= $_."***";
    }
    return $res;
}

1;
[download]

I see now that I need to set $_ as local. I find it quite counter-intuitive to have to explicitly make local the default variable. My conclusion would be that I should always make it local whenever I use it, unless I intend to use it globally... and actually I can't think of any case where using it globally would be good programming practice.

What do you expert think? Thank you for your insight!