use strict;
use warnings;
use String::Substitution qw(gsub_copy);

my $in_string = 'foo.ra';
my $find_exp  = '\.(.)(.)$';
my $replace_exp = 'b$2$1s';

my $output = gsub_copy($in_string, $find_exp, $replace_exp);
print "$output\n";  # prints: foobars