I don't see any way of modifying the algorithm of the regular expression to make it more comprehensible (maybe doing it in multiple steps?) but, when I want to make code more comprehensible, my first thought is documentation, and regular expressions can certainly take documentation. Utilizing the x modifier, the regular expression will ignore white space and comments (# delimited). See perlre. I also think uc might be more clear than your tr. Consider: 

$job_id = uc $job_id;
$job_id =~ s/^(..)(..)(..)(..)  # Invert first 4 bytes
              (..)(..)          # More comments ...
              (..)(..)
              (.{4})
              (.{12})          $
            /$4$3$2$1-$6$5-$8$7-$9-$10/x;

[download]

or the indentation/comment scheme of your choice.

In reply to Re: Rearrange hex digits. by kennethk
in thread Rearrange hex digits. by kyle

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