Your browser attempted to unzip the tarball and probably didn't do it correctly (are you using an old netscape browser?). Try

mv MIME-Base64-3.07.tar.tar MIME-Base64-3.07.tar.gz
gunzip MIME-Base64-3.07.tar.gz
tar -xvf MIME-Base64-3.07.tar
[download]

Or get the original tar.gz file using something sane, like wget (or the CPAN shell)