my %Scale_defaults = (
        -width        => 10,
        -length       => 200,
        -sliderlength => 16,
        -orient       => 'horizontal',
        -borderwidth  => 1,
        -cursor       => 'hand2',
        -showvalue    => 0
    );

# ...

my $s = $mainwindow->Scale(%Scale_defaults,
                           -showvalue => 1); # override a default