my $module_loaded;
BEGIN {
    eval {
        require Foo;
        Foo->import();
        $module_loaded = 'Foo';
        1;
    } or eval {
        require Bar;
        Bar->import();
        $module_loaded = 'Bar';
        1;
    } or do {
        # Whatever you do when you can not load
        # any of the modules you want.
    };
}