You're getting two things mixed up. The code you were using is meant to be run from the command line, not inside a script. Your script should look something like:

#!C:/Perl/bin/perl.exe -pi.bak

tr/,//d;
[download]

And run it like so:

C:\>script.pl testdata.txt
[download]

I hope this helps. This of course assumes that you just want to remove the commas and not replace them with anything else.