Hashes to the rescue!

sub intersection_length {
  my($h1, $h2) = ( { map { $_ => 1 } @{$_[0]} },
                   { map { $_ => 1 } @{$_[1]} } );
  return scalar grep exists $h2->{$_}, keys %$h1;
}
print intersection_length([ 1 .. 5 ], [ 2 .. 6 ]), $/;

__output__

4
[download]

Not optimal, but quite succinct :)
HTH

_________
broquaint