in reply to Perl beginner's issue with hash

You might be interested in Building Regex Alternations Dynamically so that you don't have to loop over your IDs.

use warnings; use strict; use Data::Dump; my @ids = ("a", "b", "c", "d", "e", "f", "g",); my ($regex) = map {qr/$_/} join '|', map {quotemeta} sort { length $b <=> length $a or $a cmp $b } @ids; my %counts = map { $_ => 0 } @ids; while (<DATA>) { if ( my ($id) = /($regex)/ ) { $counts{$id}++; } } dd \%counts; # outputs "{ a => 1, b => 2, c => 2, d => 3, e => 0, f => 3, g => 0 }" __DATA__ a b b NO ID HERE c c d d d f f f