You mean (??{ ... }). ((?{ ... }) is always zero-width.) Ok, the warning makes sense. There are (at least) two ways of hidding it:

1) Use no warnings 'regexp', or

2) refactor the regexp to hide that particular instance of the warning while leaving others on. For example,

$re0 = qr/
          (??{ $re1 })*
       /x;
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can be written as:

$re0 = qr/
          (??{ $re1 }) (??{ $re0 })
          |
          # Nothing
       /x;
[download]

and

$re0 = qr/
          (??{ $re1 })+
       /x;
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can be written as:

$re0  = qr/
           (??{ $re1 }) (??{ $re0_ })
        /x;

$re0_ = qr/
           (??{ $re1 }) (??{ $re0_ })
           |
           # Nothing
        /x;
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and

$re0  = qr/
           (??{ $re1 })?
        /x;
[download]

can be written as:

$re0  = qr/
           (??{ $re1 })
           |
           # Nothing
        /x;
[download]