You're on the right track. After this: $lines =~ s/foo/bar/gms;the substitution has been made, but it's still in memory. The next step is to write it to the file:

open(FILE, ">theother.txt")
    or die "Can't open the other file: $!\n";
print FILE $lines;
close FILE;
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You said "other file", so I'm not sure if you want to overwrite the original file or not. If you do, just substitute its name instead of theother.txt above.

HTH