Try the following:

#!/usr/bin/perl 

while (<>){
   s/CAT/mouse/g;
   print;
}
[download]

Then run the code like so:

prompt> perl -w myscript.pl 'D:\tmp\file.txt' > D:\tmp\file.txt
[download]

Update: Thanks for the reference to the -pi flags. I understand now. The while loop is superfluous with the -p and the -i will edit the file given in place. Why does he need a system() function call around the whole thing? Won't that spawn another shell?

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