I think autovivification can happen in contexts other than dereferencing an element as an lvalue. Consider the following script.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $hashref;

print "exists\n" if(exists($hashref->{foo}->{bar}));

print Dumper($hashref);
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It prints

$VAR1 = {
          'foo' => {}
        };
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So, unless the argument to exists is dereferenced as an lvalue (I don't know how it is implemented) there are other contexts.

Explaining Autovivication, Accidentally creating hash elements with grep { defined } on a hash slice and The Bad, the Ugly, and the Good of autovivification have more info.