I tried running it on ActiveState build 618, and got the same results as you-nothing. So I decided to try it from the command line with -w for warnings, and got:

C:\WINDOWS\Desktop> perl -w tk1.pl
Useless use of a constant in void context at tk1.pl line 5.
Use of uninitialized value in concatenation (.) at C:/Perl/site/lib/Tk.pm line 350.
Use of uninitialized value in concatenation (.) at C:/Perl/site/lib/Tk.pm line 350.

Line 5 was the line you have as "Mainloop;" -when I changed this to "MainLoop();", it worked for me. Trying "-w" gave only:

C:\WINDOWS\Desktop> perl -w tk1.pl
Use of uninitialized value in concatenation (.) at C:/Perl/site/lib/Tk.pm line 350.
Use of uninitialized value in concatenation (.) at C:/Perl/site/lib/Tk.pm line 350.

I did get the button you intended, then.

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