There is also another possibility besides "given", use a hash table. A simple translation like this could fit your needs:
my %ops=(add=>4, sub=>5); Instead of writing "given" code, just do a hash look up. Often replacing code with a static data table lookup is a good idea.

It is also possible to have a hash table which contains code references to subroutines. This is called a dispatch table. I just show a simple example below. There are all sorts of permutations that can be done using this theme. Setting something like this up can be relatively hard or low performance in other languages but, not so in Perl.

Have fun!

#!/usr/bin/perl

use warnings;
use diagnostics;
use strict;

my %opcodes = 
    ('add'=> sub{ print "the add opcode was passed operands: @_\n";
                  print "The opcode reported was add with hex value 4\
+n";},
     'sub'=> \&opcode_sub,
    );

while (my $instruction = <DATA>)
{
  my ($opcode, @operands) = split (/\s+/, $instruction);
  print "\ninstruction: $opcode  operands: @operands\n";
  
  print "calling $opcode...\n";
  $opcodes{$opcode}->(@operands);  # give some data to subroutine
  #$opcodes{$opcode}->();          # or no args  
}

sub opcode_sub
{
   print "The code for subtract opcode was passed these operands: @_\n
+";
   print "The opcode reported was add with hex value 5\n";
}

=outputs
instruction: add  operands: 5 6 10
calling add...
the add opcode was passed operands: 5 6 10
The opcode reported was add with hex value 4

instruction: sub  operands: 7 8
calling sub...
The code for subtract opcode was passed these operands: 7 8
The opcode reported was add with hex value 5
=cut

__DATA__
add 5 6 10 
sub 7 8
[download]