tall man writes:

Here is a start. Depending on the size of cmd, you can adjust the padding of the Z part.

use strict;

my $cc = "whatever";
# Pack a length and the cc string into a cmd field.
$cc .= "\015";
my $index = length($cc);
my $cmd = pack 'iZ10',$index,$cc;
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You can use Z* instead of Z10, so you don't have to change the number. Strictly speaking it would be better to "A" instead of "Z" since the original doesn't include the "\0", but it won't really matter since the length at the beginning doesn't include it either. There's also a bit of a short cut in recent Perl's (5.6 and later, I think):

$cc .= "\015";
my $cmd = pack "i/A*", $cc;
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which automatically takes the length for the integer.