use HELPER;: Perl looks for a file named HELPER.pm from among the paths listed in @INC, and it is found. But use HELPER; is the same as saying:

BEGIN{
  require HELPER;
  HELPER->import();
}
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The problem is that second part; you have inherited Exporter into a package named Helper. So when Perl goes looking for HELPER->import() it doesn't find anything. And what does exist, Helper->import() isn't being called, because you're using HELPER, not Helper.

You may as well be saying:

BEGIN {
  require HELPER;
  Goofy->import();
}
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Additonally, the fully qualified name of foo() isn't HELPER::foo, it's Helper::foo.