Regardless of the behavior undef may have exibited previously, the 5.005_3 perlfunc says explicitly-- Note that this is a unary operator, not a list operator.

The current perldelta also mentions the correction of this "accidental" behavior.

In reply to RE: Re: 5.6 versus 5.005 by takshaka
in thread 5.6 versus 5.005 by Ovid

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