sub get_unknown_keys (\%;) {
    my $hash = shift;

    my %allow;
    @allow{keys %$hash} = ();
    delete @allow{@_};

    return keys %allow;
}

sub func {
    my %arg = ( foo => 1, bar => 'baz', quux => 42 );
    my @allow = keys %arg;
    %arg = (%arg, @_);

    if(my @unknown = get_unknown_keys %arg, @allow) {
        carp "Ignoring unkown parameters: @unknown";
        delete @arg{@unknown};
    }

    # do stuff.
}