Earlier in the same sub in Exporter.pm, you'll see this line:

  return export $pkg, $callpkg, @_
    if $Verbose or $Debug or @fail > 1;
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So it won't make it to your snippet in question if @fail > 1. So $fail[0] seems reasonable. As for the significance of $EXPORT_OK[0], I'm as lost as you are. I don't see anywhere where it's enforced to have only one element, as with @fail. What a curious little module, Exporter.pm.

blokhead