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sub _detect_type {
  ... # logic to determine type
  return \&_open_unzipped if $rule1;
  return \&_open_zipped   if $rule2;
  return undef;
}

package Import;
use Exporter;
...
sub _open_zipped   {...}
sub _open_unzipped {...}
1;

use Test::More qw(no_plan);
use Import qw(:TEST);
...
is($test1, $result1);
is($test2, $result2);
...

is(_detect_type('unzipped.txt'), \&open_unzipped);
is(_detect_type('zipped.zip'),   \&open_zipped);