This is known limitation of Text widget in Tcl/Tk, and TIP155 proposes to fix this in Tcl/Tk, see http://www.tcl.tk/cgi-bin/tct/tip/155.html

Text is misbehaving in Tcl/Tk version 8.4, but behaves correctly in beta-version 8.5, so Tcl/Tk developers implemented that TIP.
(I just tried this -- I downloaded ActiveTCL 8.5 beta and checked this behaviour)

http://search.cpan.org/~vkon/Tcl-Tk-0.84 module from CPAN allows you to use Tcl/Tk from Perl with PerlTk syntax.
addition: ... so you can write normal code today and expect sooner or later for it to behave better after upgrading of module...

Best regards,
Courage, the Cowardly Dog