$1 and $2 are read-only, set internally by perl as a result of regex matching. If you call upper() on $foo and $bar, your function will do what you intend.

You can guard against fatal errors by wrapping in eval{}. You can catch some such errors at compile time with the (\$) prototype, but that wouldn't catch an attempt to modify $1.

Your function can be made more independent of language by writing it with uc,

sub upper {
    eval {
        $_ = uc for @_;
        1;
    }
}
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