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);