use Data::Dumper;
LINE: while (defined($_ = <ARGV>)) {
    $item = $1 if /^=item (.*)/;
    push @{$warnings{$1};}, $item if /\(W ([^)]*)\)/;
    sub END {
        print Dumper(\%warnings);
    }
    ;
}

##</code><code>##

use Data::Dumper;
while (defined($_ = <ARGV>)) {
    if (/^=item (.*)/) {
        $item = $1;
    }
    if (/\(W ([^)]*)\)/) {
        push @{$warnings{$1}}, $item;
    }
}
print Dumper(\%warnings);

##</code><code>##

use Data::Dumper;
use warnings;
use strict;
my ($item, %warnings);

while (defined($_ = <ARGV>)) {
    if (/^=item (.*)/) {
        $item = $1;
    }
    if (/\(W ([^)]*)\)/) {
        push @{$warnings{$1}}, $item;
    }
}
print Dumper(\%warnings);