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

##</code><code>##

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

&$op();

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

##</code><code>##

@safesubs = ("this_sub", "that_sub", "other_sub");
$ok = 0; #op is not okay to run until we say so ;-)
foreach (@safesubs) {
    if($op eq $_) {
        $ok = 1; #if the op is safe, set $ok to true
    }
}

if($ok) {
    &$op();
} else {
    die("The op variable does not contain a valid definition");
}