use strict; use warnings; my %hash; while () { $hash{$1}++ if /^lab(\d+)_/; } for my $lab ( sort { $a <=> $b } keys %hash ) { print "lab$lab is ", ( $hash{$lab} == 2 ? 'paired' : 'unpaired' ), "\n"; } __DATA__ lab1_set1.txt lab1_set2.txt lab2_set1.txt lab3_set1.txt lab3_set2.txt