A truly useful use of + is for constructing hashes with map: 
my %hash = map +($_=>undef), @list;

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Without the +, Perl would think you wanted to call map with 2 args, not over @list.

In reply to Re^7: Passing argument by reference (for a scalar) by etj
in thread Passing argument by reference (for a scalar) by jmClifford

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