use Benchmark;
my @a = ('x') x 1_000_000;
timethese(100, {
	'concat' => sub { my $s; $s .= $_ for @a },
	'join'   => sub { my $s = join '', @a },
},); 
__END__
    concat: 11 wallclock secs (10.86 usr +  0.01 sys = 10.87 CPU) @  9.20/s (n=100)
      join:  4 wallclock secs ( 4.01 usr +  0.00 sys =  4.01 CPU) @ 24.94/s (n=100)