There is a bug that has been outstanding since 2005:
"In Tcl/Tk 8.4 there was an API change which makes setting a window as its own master invalid.
WaitBox dies with the following error on all Tk804 builds:
can't make ".waitbox" its own master at /Perl/site/lib/Tk/Submethods.pm line 37.
at Perl/site/lib/Tk/WaitBox.pm line 58"
I tried naively commenting out the offending line in WaitBox.pm (the one that says "$cw->transient($cw->toplevel);"), and everything seems to work OK - I have used this module a lot since then, without any problems.