There'd be one package which is your actual clean namespace meant for end users, and another package containing the implementation details.

It only looks ugly because I was tired and didn't use Exporter. So with Exporter it would be like this

::::::::::::::
Foo/Internal.pm
::::::::::::::
use v5.36;

package Foo::Internal {
    use Exporter 'import';
    our @EXPORT = qw[check_it];

    use Scalar::Util 'looks_like_number';

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

1;
::::::::::::::
Foo.pm
::::::::::::::
use v5.36;

package Foo {
    use Foo::Internal;

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

1;

::::::::::::::
clean.pl
::::::::::::::
use v5.36;
use Foo;

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