Here are a few code examples that work as expected:
1. 
$hash{'a'};
print exists $hash{'a'}?'yes':'no';
#no

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2. 
$hash{'a'}=1;
print exists $hash{'a'}?'yes':'no';
#yes

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3. 
$hash{'a'}=$hash{'a'};
print exists $hash{'a'}?'yes':'no';
#yes

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4. 
sub foo{$_[0]}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#no

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5. 
sub foo{$_[0]=1}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#yes

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6. 
sub foo{$_[0]=$_[0]}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#yes

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7. 
sub foo{
    for($_[0]){
    }
}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#no

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8. 
sub foo{
    for($_[0]){
        $_=1;
    }
}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#yes

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Now, this one is, well, it's not consistent with 7..
for($hash{'a'}){
}
print exists $hash{'a'}?'yes':'no';
#yes

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This one doesn't make sense to me, does it make any to you?
sub foo{
    for($_[0]){
        $_=$_;
    }
}
foo($hash{'a'});
print exists $hash{'a'}?'yes':'no';
#no

[download]

In reply to Hash Autovivification Weirdness by Ido

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