If the level is inconsistent, you could check if there's any level with another package.

package Foo;

use strict;
use warnings;
use feature qw( say );

sub is_modulino {
   for ( my $i = 1; ; ++$i ) {
      my $caller = caller( $i );
      return 1 if !defined( $caller );
      return 0 if $caller ne __PACKAGE__;
   }
}

BEGIN {
   say is_modulino() ? 1 : 0;
}

1;
[download]

$ perl -I . -e'use A;'
0

$ perl A.pm
1
[download]

A shorter version could be obtained by unrolling the first two passes of the loop.