So this comes down to what you have "in" in terms of "bits" and what you want "out" in terms of "bits".

There are some very cool Perl functions: pack and unpack that can do a magnificent job of translation.

You will have to understand the difference between "little Endian" and "big Endian". These terms may seem strange as well as "binary coded decimal", or BCD.

I have no idea what "packed decimal" means to you. You will have to post some input and desired output.

Assuming that by "packed decimal" you mean COBOL style COMP3, try this (unverified):

sub iToComp3 {
    my $num = '' . shift; ## force to string
    ## Add padding to even length strings
    ## to accomodate trailing sign bits
    $num = '0' . $num unless length( $num ) & 1;

    my $comp3 = '';
    
    ## pack each digit into 4 bits of output
    vec( $comp3, $_, 4 ) = substr( $num, $_, 1 ) 
        for 0 .. length( $num ) - 1;

    ## Add sign bits (doesn't cater for unsigned COMP3!)
    vec( $comp3, length( $num ), 4 ) = $num < 0 ? 0x0D : 0x0C;

    return $comp3;
}
[download]

Of course! But what packed decimal on what platform? Check if Convert::IBM390 or Data::SecsPack fits your needs.

Cheers
Harry