@bKeyChars is being translated using ord() then translated back using chr(). The dashes can be added using a regex outside the first loop simplifying the outer loop. @bProductKey can be found without a need for @bDigitalProductID. The for loops can be rewritten to a more "perlish" style and we can step through @bProductKey without resorting to indexes. Finally, we can get rid of the Hungarian notation.

sub getXPkey {
    my $key     = shift;
    my @encoded = ( unpack 'C*', $Registry->{$key} )[ reverse 52 .. 66
+ ];

    # Get indices
    my @indices;
    foreach ( 0 .. 24 ) {
        my $index = 0;

        # Shift off remainder
        ( $index, $_ ) = quotient( $index, $_ ) foreach @encoded;

        # Store index.
        unshift @indices, $index;
    }

    # translate base 24 "digits" to characters
    my $cd_key =
        join '',
        qw( B C D F G H J K M P Q R T V W X Y 2 3 4 6 7 8 9 )[ @indice
+s ];

    # Add seperators
    $cd_key =
        join '-',
        $cd_key =~ /(.{5})/g;

    return $cd_key;
}

sub quotient {
    use integer;
    my( $index, $encoded ) = @_;

    # Same as $index * 256 + $product_key ???
    my $dividend = $index * 256 ^ $encoded;

    # return modulus and integer quotient
    return(
        $dividend % 24,
        $dividend / 24,
    );
}
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This makes for a little cleaner look, not a speed increase. I tend more toward clean, maintainable code than fast code.