use warnings;
use strict;

package Internal; {
    sub foo {
        my $caller = (caller(1))[3];
        my $me = (caller(0))[3];
        if ($caller !~ /^Internal::/){
            die "$me is only available internally\n"
        }

        ...
    }
}
package External; {
    sub bar {
        Internal::foo();
    }
}

package main;

External::bar();

__END__
Internal::foo is only available internally