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...
<input type="hidden" name="op" value="some_operation">
...

... #(generic code / beginning of HTML dumped here)

&$op();

... #(generic code / end of HTML dumped here)

@safesubs = ("this_sub", "that_sub", "other_sub");
$ok = 0; #op is not okay to run until we say so ;-)
...
} else {
    die("The op variable does not contain a valid definition");
}