You want to load a file from your hard dick? :-|

What you want seems to be inplace editing:

#!/usr/bin/perl -i.bak
while(<>) {
 s/trojan is like viruse/hello/g;
 print
}
[download]

The -i switch makes your script to process the files you pass as arguments to the script inplace. The extra .bak makes it create a backup.