use v5.36;

package Foo::Internal {
    use Scalar::Util 'looks_like_number';

    sub check_it ($self, $num) {
        say 'Hoorah!' if looks_like_number($num);
    }   
}

package Foo {
    sub new ($class) {
        bless {} => $class;
    }   

    no warnings 'once';
    *check_it = \&Foo::Internal::check_it; 
}

my $foo = Foo->new;
$foo->check_it(42);
$foo->looks_like_number(42);