( Please use <c> instead of <pre>. Less work for you, more features for us. )

Append the following, and you'll see why it's not an error:

print "$array_ref->[0]\n";
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You asked for a hash reference, so Perl obliged rather that giving an error. It's a very convenient (although sometimes problematic) feature called "autovivification". Solution:

use strict;
use warnings;

my $array_ref = [undef];

if (  $array_ref->[0]               # <---
   && $array_ref->[0]->{whatever}
) {
    print "true\n";
} else {
    print "false\n";
}
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Note that the arrow is optional between indexes, so
$array_ref->[0]->{whatever}
can be written as
$array_ref->[0]{whatever}