#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

# Filter out all words that can't be candidates one way or the other
my @candidates=grep { m{[RS]} } <DATA>;
chomp(@candidates);
# Sort them in order of length
my @orderedCandidates=sort{ length $a <=> length $b } @candidates;
# Push a guard on the end
push @orderedCandidates,"";

warn Data::Dumper->Dump([\@orderedCandidates],[qw(*orderedCandidates)]),' ';

my %hash;
my $length=length($orderedCandidates[0]);
for my $word (@orderedCandidates) {
    if (length($word) == $length) { # Same length ... add it to the hash
        $hash{$word}=undef;
         }
    else { # Hash is complete
        for my $Rword (grep { m{R} } keys %hash) { # Word has a R
            my $pos=0;
            while (($pos=index($Rword,'R',$pos)) >= 0) { # Found an R
                # Make its S equivalent
                my $Sword=$Rword;
                substr($Sword,$pos,1)='S';
                print "$Rword - $Sword\n"
                    if (exists $hash{$Sword});
                $pos++
                 }
             }
        # Done with this hash --- so start over
        %hash=(); $hash{$word}=undef;
        $length=length($word);
         };
     }

__DATA__
ONE
THREE
FOUR
FOUS
RRRRRRRR
RSRRRRRS
RRSRRRRR