If you want the output of a function call as part of the left-side match string in a regex, why not just assign the function output to a scalar, and use the scalar as a match string?

my $match = chr 231;
my $replc = "c";
$text =~ s/$match/$replc/gise;
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The expression content can be as dynamic, flexible and/or complicated as you need it to be -- assign different values to $match on every iteration of some loop, use some kind of data structure (hash, AoA or HoA or whatever) to store pre-computed (or hard-coded) pairings of $match and $replc, and so on. Check out the "qr" operator in "perldoc perlop" for more ideas...

The only reason to want function calls in the matching part would be to write more compact code, but "more compact" often means "harder to read/debug/maintain". (I guess another reason to want this would be to write "better" obfus.)

Using executable code in the replacement part is of course a different story -- you can do lots of things with this that would be really hard to do any other way.