my $count = do {
    my %seen;
    @seen{@somenames} = ();
    my $c;
    foreach my $name ( @allnames ) {
      $c++ if exists $seen{$name};
    }
    $c;
};

##</code><code>##

my $count;
foreach my $name (@allnames) {
  $count++ if grep { $_ eq $name } @somenames;
}