I would probably have used 
(my $final = $start) =~ s/(.)(.)/$2$1/g;

[download]
which has the advantage of displaying a really nice pair.
Caution: Contents may have been coded under pressure.

In reply to Re: Swapping odd-even characters of BCD hex data by Roy Johnson
in thread Swapping odd-even characters of BCD hex data by polettix

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