how about:

arg.pl '\x61' '\x41'

$arg_2 =~ s@\\x([0-9a-fA-F]+)@chr hex $1@ge;
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thanks, I was trying to use chr after Corion's suggestion to use pack/unpack, but I couldn't figure out how to make it work with hex (easily).

EDIT: added a-fA-F classes.