That's bad advice. You should never have to use exists on an array element. It returns useless information and doesn't solve the problem you claims it solves (as explained by almut).

See modules autovivification and warnings::autoviv. (Well, the latter doesn't exist yet. Working on it.)

and will not auto-vivify them

defined will not autovivify either:

my @array;
if (defined $array[20]) { }                           # no autovivific
+ation, thus
if (defined $array[20]) { warn "[20] does exist!" }   # no warning
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Typically, it's a dereferencing operation that unexpededly autovivifies.  exists shares the same problem:

my @array;
if (exists $array[20]->[42]) { }
if (exists $array[20]) { warn "[20] does exist!" }

__END__
[20] does exist! at ./808911.pl line 5.
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I should have been more clear - defined does not distinguish between "never existed" and "is undef" for hash and array elements. I therefore consider exists a better test for unexpected auto-vivification.

defined does not distinguish between "never existed" and "is undef" for hash and array elements.

Neither does exist. It checks if the element currently exists, not if the element has ever existed.

Either way, so what?

I therefore consider exists a better test for unexpected auto-vivification.

That makes no sense. It doesn't prevent autovivification. It doesn't tell you whether something was autovivified or not. It doesn't help at all.

$ perl -le'{
    my @a;  $a[15][0];
    print "After autovivification:      ",
        exists($a[14])  || 0,
        exists($a[15])  || 0,
        defined($a[14]) || 0,
        defined($a[15]) || 0;
}{
    my @a;  $a[15] = [];
    print "After explicit vivification: ",
        exists($a[14])  || 0,
        exists($a[15])  || 0,
        defined($a[14]) || 0,
        defined($a[15]) || 0;
}'
After autovivification:      0101
After explicit vivification: 0101
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