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

package Import;
use Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(load_file);
@EXPORT_OK = qw(_detect_type
                _open_zipped
                _open_unzipped
               );
%EXPORT_TAGS = {STD  => \@EXPORT,
                TEST => \@EXPORT_OK
               };

sub load_file {
  my $filename = shift;
  my $openner = _detect_type($filename);
  &$openner($filename);
  while (my $line = <HANDLE>) {
    ... # do stuff with lines
  }
}

sub _detect_type   {...}
sub _open_zipped   {...}
sub _open_unzipped {...}
1;

##</code><code>##

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

##</code><code>##

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