Curiously enough perldoc -f defined has something to say about this: 
Use of "defined" on aggregates (hashes and arrays) is depre-
cated.  It used to report whether memory for that aggregate has
ever been allocated.  This behavior may disappear in future
versions of Perl.  You should instead use a simple test for
size:

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In reply to Re: if (defined @arr) problem by blazar
in thread if (defined @arr) problem by jeanluca

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