You can only use a simple scalar variable as a foreach index.

However, there are ways to "subvert" this, if you're determined. Here are two possible scenarios (by no means they are perfect - there can be issues with dynamic scoping (local)).

If you don't need aliasing:

{
  local $common::loaded{junk};
  for (@modules) {
    $common::loaded{junk} = $_;
    # ...
  }
}
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If you need aliasing (requires Array::RefElem):

use Array::RefElem 'hv_store';

{
  local $common::loaded{junk};
  for (@modules) {
    hv_store %common::loaded, 'junk', $_;
    # ...
  }
}
[download]

Anyway, this is bad advice ;)