use Benchmark qw/cmpthese/;
my $append = 'B' x 50;
cmpthese (100, {
  dot    => sub { my $c = 'A' x 50; $c = $c . $append                for 1..1e5; },
  dot_eq => sub { my $c = 'A' x 50; $c .= $append                    for 1..1e5; },
  substr => sub { my $c = 'A' x 50; substr $c, length $c, 0, $append for 1..1e5; },
});
__END__
         Rate substr    dot dot_eq
substr 16.5/s     --   -35%   -37%
dot    25.5/s    55%     --    -2%
dot_eq 26.0/s    58%     2%     --