use Benchmark qw(cmpthese);

cmpthese(-1, {
    'ampersand' => 'use constant foo => 2; my $x = 0; for (1..10000) { $x += &foo } ',
    'no_amp'    => 'use constant foo => 2; my $x = 0; for (1..10000) { $x += foo } ',
});
__END__
           Rate ampersand    no_amp
ampersand 570/s        --      -39%
no_amp    939/s       65%        --