Very simply. Evaluate the array in scalar context.

$size = @array;
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Remember that the size of an array is not the same as the last element of an array. Since indexing of elements starts at zero, the last element of a ten-element array is 9. See the following code:

my @array = qw/1 2 3 4 5/;
print scalar @array, "\n";  # Size of array
print $#array, "\n";        # Last element of array
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And the output:

5
4

Originally posted as a Categorized Answer.