Try this with your dictionary file. If performance is a problem break into separate files according to word length.

#!perl
use strict;

open IN,'dict.txt' or die "$!";
my %dict=();
for (<IN>){
  chomp;
  $dict{uc $_}=1;
};
close IN;

for my $word (sort keys %dict){
  next unless $word =~ /R/;
  my @f = split //,$word;
  
  # loop over each letter
  # changing R to S 
  # to create new word in $w
  for my $i (0..$#f){
    if ($f[$i] eq 'R'){
      my $w = $word;
      substr($w,$i,1) = "S";
      # check if generated word exists in dict
      if (exists $dict{$w}){
        print "$word $w\n";
      } 
    }
  }
}
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