How would you define an interface where you specify a string and a place where to store the value if that string is matched somewhere?

You pass a list of pairs to GetOptions(...), and that list of pairs is 1) the specification of the switch ("name|alias=value type") and 2) the place where the value should be written to. The way to pass a place around in Perl is usually to pass a reference to the target around, which is what happens with the \$foo thing.

An alternative approach is to make GetOptions(...) store all things it finds in a hash. This will give you a plain hash without weird syntax, but on the downside, you lose the typo checking that strict.pm provides.