Any element of any array can be "undefined".
A reference to an array is defined if the array exists. That array may contain Zero (No) elements.

#!/usr/bin/perl -w
use strict;

my $arr_ref2 = [];

print "".defined $arr_ref2 ? "arr_ref2 is defined\n" 
                           : "arr_ref2 is not defined\n";
                                                     
print "".defined $arr_ref2->[0] ? "arr_ref2->[0] is defined\n" 
                                : "arr_ref2->[0] is not defined\n";
                                
my $arr_ref3 = [1,2,undef,4];   # [2] is undefined!
print "\n";
print "".defined $arr_ref3->[2] ? "arr_ref3->[2] is defined\n" 
                                : "arr_ref3->[2] is not defined\n";
print "".defined $arr_ref3->[1] ? "arr_ref3->[1] is defined\n" 
                                : "arr_ref3->[1] is not defined\n";
                               

__END__
arr_ref2 is defined
arr_ref2->[0] is not defined

arr_ref3->[2] is not defined
arr_ref3->[1] is defined
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