use feature qw(state); # Perl 5.10+
use Type::Params qw(compile);
use Types::Standard qw(Num);

sub add {
  state $check = compile( Num, Num );
  my ($x, $y) = $check->(@_);  # will croak if not passed two numbers
  
  my $sum = $x + $y;
  
  Num->assert_return( $sum );
}