#!/usr/bin/perl

use strict;  
use warnings;

my $file = "/usr/share/dict/words";

open my $fh => $file or die;

my %seen;

while (<$fh>) {
    chomp;
    if (/m$/) {
        $seen {+lc} ++;
        next;
    }
    elsif (/rn$/) {
        if ($seen {lc "$`m"}) {
            print "$`m/$_\n";
        }
        next;
    }
}
__END__
bum/burn
chum/churn
dam/darn
hom/horn
modem/modern
stem/stern
tom/torn