my $sign_encd = MIME::Base64::encode_base64( $sign->serialize );
to re-create signature:
my $sign = Crypt::DSA::Signature->new( Content=>$sign_encd );
notice - you don't need to 'MIME::Base64::decode_base64' the Content. The 'Crypt::DSA::Signature->new' anticipates the Content to be 'encode_base64'ed. It probably will work on not encoded Content but I did not test this.
The author uses Math::BigInt throughout the module although he says 'The implementation itself is pure Perl, although the heavy-duty mathematics underneath are provided by the Math::Pari library'.