I propose the config file change. I trust that the given-when construct is a different animal inside than switch-case.

It is a switch statement

given ($convention) {
    when ('E') {
        print "east longitude $long radians\n";
    }
    when ('W') {
        $long = TWOPI-$long;
        print "west longitude $long radians\n";
    }
    default {
        print "error: $convention not valid\n";
    }
}
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But it doesn't have to do its own Perl parsing like Switch because it's a builtin control statement like for and while.