The mentioned error is because you didn't export doubler. See Exporter for details. First, make it exportable:

our %EXPORT_TAGS = ( 'all' => [ qw(
    doubler
) ] );
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Second, import it. A normal script would import it as follows:

use Integer::Doubler qw( doubler );
  -or-
use Integer::Doubler qw( :all );
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But a test script uses use_ok as follows instead in order to report compile issues in the module more nicely:

BEGIN {
    use_ok 'Integer::Doubler', qw( doubler );
      -or-
    use_ok 'Integer::Doubler', qw( :all );
}
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Finally, I wanted to mention an issue you haven't encountered yet. Don't print from your tests. Use the functions in Test::More instead. You'll use is almost exclusively, using ok only when is isn't sufficient (since is gives more info on failures).

For example,

print "2 * 2 =", doubler(2);
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should be

is( doubler(2), 2*2, 'doubler(2)' );
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(The last argument is a short description of the test.)

Now that you have two tests (use_ok and is), you need to tell that to the test harness.

use Test::More tests => 2;
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