The correct way to check for the presence of a key is to use exists. You'll note, though, that these keys exist since they're from the keys call in the foreach. The concern is that the value will be undefined which is going to generate warnings if used.

Now regarding this so-called autovivification. People get so paranoid about that sort of thing that they don their tin-foil hats at the merest hint of trouble. It's a lot harder to do than you think. Merely thinking of a key is not enough: 
use Data::Dumper;

my $foo = {};

1 if (defined($foo->{bar}));
print Dumper($foo);

1 if (defined($foo->{bar}->{baz}));
print Dumper($foo);

[download]
The second one causes autovivification since you're actually extending some pseudo-predefined structure. You can't dereference an undefined value, so it just makes one up for you.

In reply to Re^2: DBD::mysql Unusual Behavior by tadman
in thread DBD::mysql Unusual Behavior by tadman

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