The other annoying warning is "Name ... used only once: possible typo.
+..". For this I do this song-and-dance:

  $DB::single = $DB::single = 1;
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There's no warning if the variable already exists, which seems correct to me. This does not warn.

    use DB_File;
    # Just some global in DB_File.
    $DB_File::db_version = 42;
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This does warn.

    use DB_File;
    $DB_File::wibble = 42;
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PS The warning happens at compile time so the variable has to exist at compile time. This means:

    require DB_File;
    $DB_File::db_version = 42;
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does warn.