or download this
cpan[5]> install Tk
...
Running make for S/SR/SREZIC/Tk-804.029.tar.gz
Catching error: "mkdir /Users/fenimore/.cpan/sources/authors/id/S/SR:
+Permission denied at /Library/Perl/Updates/5.12.3/CPAN/FTP.pm line 51
+9\cJ" at /Library/Perl/Updates/5.12.3/CPAN.pm line 392
CPAN::shell() called at -e line 1