Usually I have user input for searching a database, so in almost all cases I can exclude non-word characters. But I have also some scripts the user can influence by his input, in a way that one subroutine is called based on the input in a select box. But I never call the subroutine directly from the users input, but use a construct like the following

<snip>
if ($user_input eq "do this") {
  &sub_a;
}
elseif ($user_input eq "do that") {
  &sub_b;
}
else {
  &error("","unexpected input","The input you supplied is not supporte
+d");
}
</snip>
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Strictly speaking this is not untainting. If this is insecure in any way, please inform me.
Thanks

neophyte