Dividing by @_ (which in scalar context gives the number of elements) returns the average.

What happens if the array is empty (0 elements)?

Maybe:

>perl -wMstrict -le
"use List::AllUtils qw(sum);
 sub mean { return @_ ? sum(@_) / @_ : 0 }
 my @Array = (3, 5, 7, 12,24);
 print mean(@Array);
 print mean();
"
10.2
0
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Update: Per oha: What is the proper result of averaging zero elements? 0? undef? A divide-by-zero (or some other) exception? Something else? Any of these results can easily be supported. I wanted to make the point that this boundary condition should at least be considered.