I did previously a one liner test without Tk and couldn't reproduce the issue, that's why I added Tk which is also used in the program where I originally encountered this issue. Therefore I have a feeling that Tk has something to do with this issue.

I noticed one other strange behavior with MacPorts Perl-Tk, it only seems to accept screen units in pixels, not inches, don't know what's up with that, but that's a different not so important (for me) issue.