#earlier in the script, we have created
#the array of files to edit, and named it @files

foreach my $filename (@files) {
 my $backup = $filename . '.bak';
 copy($filename, $backup) 
  unless -e $backup;
 open(FILE2CHANGE, "<$backup");
 open(UPDATED, ">$filename") or die "$filename: $!";
 while (<FILE2CHANGE>) {
  s/foo/bar/g;
  print UPDATED;
 }
 close FILE2CHANGE;
 close UPDATED;
}