$char_lpar and $char_rpar get interpolated inside your substitution. You can quote them using the \Q\E construct:

$data =~ s/\Q$char_lpar\E/sprintf("%.4x;", $decval_1)/ge;
$data =~ s/\Q$char_rpar\E/sprintf("%.4x;", $decval_2)/ge;
[download]

-- Frank