Agreed, this sounds like a good place for require.

This would also let the files share common base names with different extensions, as the OP had desired.

  # In lib/MyCode/Form1.pm
  package MyCode::Form1;

  require "lib/MyCode/Form1.tkfrm";

  sub handle_button { ... }

  1;
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  # In lib/MyCode/Form1.tkfrm
  # package declaration is optional

  sub layout_form { ... }

  1;
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