If I recall correctly (it's been a while since I wrote any Perl/Tk applications), the dithering is handled internally by the Tk code itself, leaving you very little control over it.

For a question this detailed and specific, I would recommend contacting the module's maintainer (e-mail address should be in the documentation), or joining the Perl/Tk mailing list. Info on that list is on the page for the Perl/Tk Module List.

--rjray