No difference between exists and defined in this regard. In both cases, the *intermediate* levels will spring into existance. The last level isn't dereferenced as a hash.

If you have perl installed, you can try it out yourself: 

$ perl -e 'use Data::Dump; exists $a->{foo}{bar}; dd $a;'
{ foo => {} }
$ perl -e 'use Data::Dump; defined $a->{foo}{bar}; dd $a;'
{ foo => {} }

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In reply to Re^3: Strange memory growth by Anonymous Monk
in thread Strange memory growth by spica1001

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