% cat t0
use strict;
use warnings;

my $input = 'foo.ra';
my $find = '\.(.)(.)$';
my $replace = 'b$2$1s';
printf qq{result "%s"\n}, replace($input, $find, $replace);
exit 0;

sub replace {
  my($input, $find, $replace) = @_;
  $input =~ s{$find}{qq{"$replace"}}ee
      or die "input did not match";
  return $input
}
% perl t0
result "foobars"
%