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my $string = "abababa";
$string = reverse $string;
$string =~ s/b/x/;
$string = reverse $string;
print $string,$/;

my $string = "abababa";
my $ri  = rindex( $string, 'b' );
substr( $string, $ri, 1) =  'x';
print $string,$/;

use Benchmark 'cmpthese';

...
neglook 719252/s      --    -19%    -20%
rindex  888889/s     24%      --     -1%
sexeger 901442/s     25%      1%      --

use Benchmark 'cmpthese';
use strict;
...
sexeger  884695/s     48%     23%      0%      --    -71%
neganch 3003003/s    404%    319%    239%    239%      --