Huh? Transparent background? What would you want to see through that?
Or is it the "foreground" that is non-transparent? Then perhaps your first step (html2ps or ps2pdf) explicitly sets the background color to "white". Try to omit that.

Update: from what I could google about html2ps, it defaults to a white background, so you'd have to modify your html2psrc-tst file.
If you're doing this anyway, perhaps you can integrate the new background there, so that you eliminate the need for PDF::Tk …