For the substr part, why not just:

(substr ("0$1", -2))
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Update: In fact I don't see any reason for the second bracketed expression in the regex. Also I can't see how it expands the first hex number if that's necessary. Is this better? or have I missed something subtle?

$str =~ s/(\.?[0-9a-eA-E]*)/(substr("0$1", -2))/ge;
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Nuance