use Benchmark qw<cmpthese>;

my $string      = 'Long string' x 1000;
my $really_long = $string x 100;
my $ref         = \$string;
my $long_ref    = \$really_long;

cmpthese(-5, {
    map { $_ => "length($_)" } qw< $string $$ref $really_long $$long_ref >
});
__END__
                   Rate   $$long_ref        $$ref $really_long      $string
$$long_ref   17305170/s           --          -9%         -36%         -38%
$$ref        18937129/s           9%           --         -30%         -32%
$really_long 27185304/s          57%          44%           --          -2%
$string      27870647/s          61%          47%           3%           --