I agree, hashes seem to be more appropriate for this kind of thing, I've whipped up a generic sub here that seems to be a little more in tone with what the original poster requested:

#!/usr/bin/perl -w

use strict; #always

sub myencode {
  my ($sentence,$encodehash)=@_ or die "not enough parameters for enco
+de:@_";
  foreach my $key (keys %$encodehash) {
    $sentence =~ s!$key!$$encodehash{$key}!g;
  }
  return $sentence;
}
my %encoding=("A"=>"D","B"=>"E","C"=>"F");
print myencode("ABC",\%encoding);
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qw[marcus]