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]