I am looking for a way to run all occurrences of a certain function in a file.

I don't get what you are up to. Run the file, and all ocurrences of the function will eventually been run... what is your __x() function about?

Maybe you are looking for a way to wrap functions?

if ($debug) {
    my $sub = \&some_function;
    my $wrapsub = sub { __x(@_); $sub->(@_) };
    *some_function = $wrapsub;
}
[download]

Then, whenever some_function() is invoked, your __x() is called first.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}