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);

##</code><code>##

use Perl6::Slurp;        #<= only handles input at present
use File::Slurp;         # permits writing.

# and then.... 
....
 $text = slurp($file);
 $text =~ s/$this/$that/smg;
... with renaming here ? (see above).
 write_file($file, $text);