&foo(
    bork  => 'glorp',
    ningi => 'fertangg!',
    flarp => undef
);

exit;

sub foo(%) {
    my %args_of = @_;

    foreach ( keys(%args_of) ) {
        print "The argument '", $_, "' was set to: ",
          defined( $args_of{$_} )
          ? $args_of{$_}
          : '(undefined)', "\n";
    }
    1;
}