You would need /e flag (see L<perlop(1)> for s///) to evaluate the right hand side. Secondly, function is chr() not "char()" to change a given number to a character.

UPDATE: Just noticed that the part of the regex in OP is supposed to be [0-7][0-9A-F] not [0-70-9A-F], otherwise OP could not have gotten "61" in output.

Edit: g0n removed TT tags & replaced POD style code markup with code tags