use strict;
use warnings;

my $MATCH_THIS = q(aa);
my @filenames = qw(file1:aa file2:bb file3:cc file4:AA);

for my $fn (@filenames) {
    print "Matched $fn\n" if $fn =~ /$MATCH_THIS/i;
}

##</code><code>##

Matched file1:aa
Matched file4:AA