The texfiles.pl program is a Perl script and as such for it to work you need to have the Perl system installed on your computer. Perl is freely available for virtually every platform. If you use Windows you can get the ActiveState Perl distribution at their download site. If you are on Linux you probably already have Perl installed, but if not, go to the CPAN site which has pointers to distrubutions for many other platforms including Linux.