use constant foo => 'bar';

sub crack {
    print &foo, $/;
}

crack(1, 2, 3);

##</code><code>##

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%        --

##</code><code>##

cmpthese(-1, {
    'ampersand' => '
            use constant foo => 2; 
            sub a { my $x = 0; for (1..10000) { $x += &foo } }
            a(%ENV, %ENV, %ENV)
        ',
    'no_amp' => '
            use constant foo => 2; 
            sub a { my $x = 0; for (1..10000) { $x += foo } }
            a(%ENV, %ENV, %ENV)
        ',
});
__END__
           Rate ampersand    no_amp
ampersand 330/s        --      -61%
no_amp    849/s      157%        --