Inline::C allows a pretty fast implementation...

use 5.010;
use Benchmark qw(cmpthese);
use Inline C => <<'INLINE';
int count1 (SV* name1, ...)
{
   Inline_Stack_Vars;
   return Inline_Stack_Items;
}
INLINE

sub count2 { scalar @_ }
sub count3 { 0 + @_ }
sub count4 { my $_ = @_ }
sub count5 (\@) { scalar @{$_[0]} }

my @array = 'a'..'z';

my %implementations = (
    count1 => sub { count1(@array) },
    count2 => sub { count2(@array) },
    count3 => sub { count3(@array) },
    count4 => sub { count4(@array) },
    count5 => sub { count5(@array) },
);

for my $i (sort keys %implementations) {
    say $i, ": ", $implementations{$i}->();
}

cmpthese(250_000, \%implementations);
[download]

XS may well allow faster still.