use File::Slurp:
# get one file here, as $file
my($bakfile, $file, $text); 
# maybe set up file renaming with something like this...
($bakfile = $file ) =~ s/\.([^.]+)$/\.bak/;
# then, the main bit
 $text = read_file($file);
 $text =~ s/this/that/smg;
 rename($file, $bakfile);
 write_file($file, $text);