mosh,
I am going to forget for a second you said IP address and that you want to use pack.

my $new = join '', map { sprintf('%.2d', $_) } split /\./, $ip;
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If you really want the spaces just change the first argument to join. I am not great at pack/unpack so I use what I know works. I question what you would want to happen if you have 3 digit octets?