Sometimes a closure is an object waiting to happen. (And sometimes the reverse.) Consider:

package Encoder;

## Basic constructor
sub new {
   my $type = shift;
   my $self = {};
   bless $self, $type;
}

## Define the common interface
sub shared_stuff { return reverse $_[1]; }

## Define the uncommon interface
sub other_stuff { 
   die "Somebody didn't define the other stuff!"; 
}

## Generalized encoding method
sub encode {
   my $self = shift;
   my ($text) = @_;

   $self->common_stuff($text);
   $self->other_stuff($text);
}
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That defines your base class, where all the common code can live.

Then, for each kind of encoder, you can define a subclass. For many subclasses, they'll be doing the same overall thing with variations, and you can just redefine the 'other_stuff' method. Otherwise, you can redefine 'encode' itself... but you'll still have 'common_stuff' to call if you want it.

# In Encoder/B.pm
package Encoder::B;

use base 'Encoder';

sub other_stuff {
   my ($text) = @_;

   my $new_text = $text;
   $new_text =~ s/a/c/g;
}
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Then, finally, you only need a factory subroutine to create the right type of encoder for each user preference:

use Encoder::B;
use Encoder::Whatever;

our %Encoder_Table => (
    'b' => 'Encoder::B',
    'c' => 'Encoder::Whatever',
);

sub encoder_factory {
  my ($e) = @_;

  exists $Encoder_Table{$e} 
    or die "'$e' is not a valid type of encoder, sorry!";

  return $Encoder_Table{$e}->new();
}
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And, finally, to get your new encoder and use it:

my $encoder = encoder_factory($whatever_e_is);
print $encoder->encode("Hahahaha!");
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There will be a speed hit, but it'll be faster than doing constant per-word checks, and easier to keep organized than closures-upon-closures-upon-closures.

Note: Code is untested and I've been drinking Cabernet.

stephen