It struck me that the way you have implemented your degrade, if one of the rgb value is less than the value by which you are degrading $rgbarray[$rgblowcount] >= $degrade;, it is left untouched. This seems wrong to me.

Say you had the rgb value

#ff7fff

a bright purplish pink and you degrade it by 128 (0x80) then using your algorithm it becomes

#7f7f7f

a mid-grey.

Wouldn't it be better to degrade all of the triplet evenly until they reach zero thereby maintaining the relative proportions of the three colours as long as possible?

Using my example above,

#ff7fff

degraded by 128 becomes

#7f007f

a darker purple colour.

To that end, I offer...

#! perl -sw
use strict;

sub min{ $_[0]<$_[1] ? $_[0] : $_[1] }

sub degrade{
    my ($rgb, $degr) = (hex(shift), pop);
    $rgb -= min( $rgb&(0xff<<$_), $degr<<$_ ) for (0,8,16);
    sprintf '%06x', $rgb;
}

my $rgb = "FF9944";

printf 'Degrading %6s by %3d gives %6s%s', $rgb, $_, degrade( $rgb, $_
+), $/
    for (0x01, 0x0f, 0x10, 0x44, 0x80, 0xcc, 0xee, 0xf0);

__END__
C:\test>204824
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