Note also that in deeply nested hashes, exists will autovivify intermediate levels of the hash in the process of testing the existence of a low level element (update: but see this for further clarification).

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my %hash;
 print 'yes' if exists $hash{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd \%hash;
"
{ www => { xxx => { yyy => {} } } }
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This effect is also avoided with  no autovivification; enabled.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"no autovivification;
 ;;
 my %hash;
 print 'yes' if exists $hash{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd \%hash;
"
{}
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Update 1: See also the recent discussion threads Sometimes undef is initialized and sometimes not when hash values are fed to grep and unexpected modify hash in a distance with grep { $_ } - both by the same monk!

Update 2: To illustrate the behavior noted by haukex here, consider:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my %h;
 ;;
 $h{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd 'access in void context', \%h;
 %h = ();
 dd 'assigning empty list really does clear hash', \%h;
 ;;
 my $x = $h{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd 'access in assignment (rvalue) context', \%h;
 %h = ();
 ;;
 1 if $h{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd 'access in boolean context', \%h;
 %h = ();
 ;;
 1 for $h{'www'}{'xxx'}{'yyy'}{'zzz'};
 dd 'access in aliased (lvalue) context', \%h;
"
Useless use of hash element in void context at -e line 1.
("access in void context", { www => { xxx => { yyy => {} } } })

("assigning empty list really does clear hash", {})

(
  "access in assignment (rvalue) context",
  { www => { xxx => { yyy => {} } } },
)

(
  "access in boolean context",
  { www => { xxx => { yyy => {} } } },
)

(
  "access in aliased (lvalue) context",
  { www => { xxx => { yyy => { zzz => undef } } } },
)
[download]

In every case except the for-loop (i.e., rvalue accesses), intermediate elements are created but not the lowest-level 'zzz' element. In the for-loop case in which aliasing creates an lvalue access context, the 'zzz' element is created.