I wrote a desktop GUI program a few years back called gRMA (http://www.tregar.com/gRMA) using Perl and Tk.pm. By the end of the project I was wishing I'd used TCL/Tk instead. I love Perl, but TCL's binding to Tk is considerably cleaner. Also, my TCL style is considerably simpler than my Perl style, which tends to be 90% OO. All things considered, I think gRMA would have been a better program in TCL/Tk.

But I've had more experiences where I built something in another language and later realized that I could have done it in half the time in Perl.

-sam