It's amazing how something so minor can turn Perl from a happy, simple programming language into an evil cancerous growth that consumes all available memory on your system. Not even strict or -w could save you.

It all started with some code that used used to extract a particular value from a simple HoH: 
my $foo = { foo => { bar => 'baz' } };

foreach (keys %foo)
{
        if (defined($foo->{$_}->{foo})
         && defined($foo->{$_}->{foo}->{bar}))
        {
                return $foo->{$_}->{foo}->{bar};
        }

        return;
}

[download]
To prevent auto-vivification, it tests carefully and returns the appropriate value. This worked well in practice, but in order to make the function more useful, it now had to search an AoHoH and return the first match. A simple change, of course. How about this: 
my $foo = [
        { foo => { bar => 'baz' } },
];

foreach my $foo_entry (@$foo)
{
        foreach (keys %$foo_entry)
        {
                if (defined($foo->{$_}->{foo})
                 && defined($foo->{$_}->{foo}->{bar}))
                {
                        return $foo->{$_}->{foo}->{bar};
                }
        }

        return;
}

[download]
Can you spot the error? strict can't. When I run this, I'm into swap space in a matter of seconds, and...then...things...get...very...slow.

Somehow I messed up the variable names, since in the actual source they were slightly more similar and there was more clutter which prevented me from seeing this right away.

If the code is "fixed", it runs fine.

What is strange is that this is all caused by using a AoHoH instead of a AoH. An example using just AoH causes a warning: 
my $foo = [
        { foo => 'bar' },
];

foreach my $foo_entry (@$foo)
{
        foreach (keys %$foo_entry)
        {
                if (defined($foo->{$_}->{foo})
                 && defined($foo->{$_}->{foo}->{bar}))
                {
                        return $foo->{$_}->{foo}->{bar};
                }
        }

        return;
}

[download]
Something is falling through the cracks here, no?

In reply to Automagic Array to Hash Conversion? Pseudo-Hash Explosion by tadman

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