It depends what you're doing really. In your example, join beats concat, but that's because you are looping through an array. Here though, concatenation beats it (albeit only by about 10%):

use strict;
use warnings;
use Benchmark qw{ cmpthese };

sub foo { 'abc' };
sub bar { 'def' };

cmpthese( -3, {
    concat => sub { my $ret = foo().bar(); return $ret; },
    join   => sub { my $ret = join q{}, foo(), bar(); return $ret; },
});
[download]

And here (thanks to the Perl compiler being so good at constant folding) concatenation is more than twice as fast as join:

use strict;
use warnings;
use Benchmark qw{ cmpthese };

sub foo () { 'abc' };
sub bar () { 'def' };

cmpthese( -3, {
    concat => sub { my $ret = foo . bar; return $ret; },
    join   => sub { my $ret = join q{}, foo, bar; return $ret; },
});
[download]