None of them will.

{
   my %h;
   1 if exists $h{key};
   print("exists:  ", (%h ? "auto-vivi" : "empty"), "\n");
}
{
   my %h;
   1 if defined $h{key};
   print("defined: ", (%h ? "auto-vivi" : "empty"), "\n");
}
{
   my %h;
   1 if $h{key};
   print("true:    ", (%h ? "auto-vivi" : "empty"), "\n");
}

__END__
exists:  empty
defined: empty
true:    empty
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The following are the only forms of auto-vivification that come to mind:

# If $h{$i} and $a[$i] are not defined,

$h{$i}{$j}  # Creates a  hash  and stores a ref to it in $h{$i}.
$h{$i}[$j]  # Creates an array and stores a ref to it in $h{$i}.
$a[$i]{$j}  # Creates a  hash  and stores a ref to it in $a[$i].
$a[$i][$j]  # Creates an array and stores a ref to it in $a[$i].
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Thanks for the info. The exists documentation does warn that your first example of autovivification ($h{$i}{$j}) will autovivify even if you're checking with exists. That is,

{
   my %h;
   1 if exists $h{foo}{bar};
   print("exists 2:  ", (%h ? "auto-vivi" : "empty"), "\n");
}
[download]

prints "auto-vivi".

But for some reason I thought it was easier to autovivify keys.

Keys are never auto-vivified. They are always created through assignment (incl the implicit assignment of an autovivified hash or array as shown in an earlier post).

$h{$k} = $v;
@h{$k} = ($v);
%h = ($k, $v);  # Also removes everything else.
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.