To fix up the case sensitivity issues, we can do something like what I did here. A new rewrite of the code, will display all matches (2 or more), that follow the 'rn'/'m' rule. For example, 'stern', 'Stern', and 'stem' are all in my dictionary. S it displays all three :)

#!/usr/bin/perl -w
use strict;

open my $dict, '<', '/usr/share/dict/words'
  or die "open failed: $!";

my %words;

while (<$dict>) {
  chomp $_;
  my $index = lc $_;
  next unless $index =~ s/(m|rn)\z//i;
  push @{$words{$index}{$1}}, $_;
}

while (my ($index, $word) = each %words) {
  print join(" / ", @{$word->{rn}}, @{$word->{m}}), "\n"
    if $word->{rn} && $word->{m};
}
[download]