BEGIN {
   my @fields = qw/field1 field2/;
   use fields @fields;
}
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is equivalent to

BEGIN {
   my @fields = qw/field1 field2/;
   BEGIN {
      require fields;
      fields->import(@fields);
   }
}
[download]

which is compiled and executed as follows:

1. my @fields = qw/field1 field2/; is compiled
2. require fields; is compiled
3. fields->import(@fields); is compiled
4. require fields; is executed
5. fields->import(@fields); is executed
6. my @fields = qw/field1 field2/; is executed

As you can see, an empty array is passed to the module. I believe you were mistaken when you claimed it worked.