If you are on a 32-bit system, you must use Math::BigInt. The following returns 2348273369088.

#!/usr/bin/perl
use strict;
use warnings;
use integer;
use Math::BigInt;

my %iupac_dgn;
my @iupac_code = ('M','R','W','S','Y','K','V','H','D','B','N');
my @iupac_den = ('2','2','2','2','2','2','3','3','3','3','4');     
@iupac_dgn{@iupac_code}=@iupac_den;

my $string = 'GGNMDNNSNNNNDBNVWVSMNNHYNBNG'; 
my @residues = split(//, $string);
my $degeneracy = Math::BigInt->new( 1 );

foreach my $residues(@residues){
    $degeneracy *= $iupac_dgn{$residues} || 1;
}

printf "%28s, %30s\n", $string, $degeneracy;
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Update: I will concur with Tye as long as the result is less than 2 ** 49 on a 32b machine/implementation. Thanks.

Good Luck. -c