use v6.d;

sub go(Int $x where 1 < $_ < 9) {    # $_ refers to $x
    say $x;
}

go(3);
go(0);

--output:--
3
Constraint type check failed in binding to parameter '$x'; expected anonymous constraint to be met but got Int (0)
  in sub go at b.raku line 4
  in block <unit> at b.raku line 9