use Benchmark qw/cmpthese/;
my $append = 'B' x 256;
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_eq    dot
substr 6.99/s     --   -35%   -49%
dot_eq 10.7/s    53%     --   -21%
dot    13.6/s    95%    27%     --