sub Open {
        my $opts= UNIVERSAL::isa( $_[-1], "HASH" )
            ? pop(@_) : {};
        my( $file, $mode, $mask )= @_;
        # ...
    }

    Open( "file" );
    Open( "file", { Exclusive=> 1 } );
    Open( "file", "w", 0666, { Locked=>1 } );