sub FETCH { my ($this, $key)=@_;
 return $this->{'l'}{$key} if exists $this->{'l'}{$key};
 print "trying f's $key: $this->{f}{$key}\n";
 $this->{'l'}{$key}=eval($this->{f}{$key});
 return $this->{'l'}{$key};
}
Can't use an undefined value as a HASH reference at t2.pl line 27.<BR>
+<BR>
Why does pwotie's FETCH work ok with $o{0} called first, but fail when
+ $o{0}->{hi} is called first?
[download]

It doesn't work, because $o{0} is undefined, and you're trying to use that as a hash reference. Remember that when you use a tied hash, you can't count on complex vivification. You'll have to initialise a hash ref yourself:

sub FETCH {
    my ($this, $key) = @_;
    return $this->{l}{$key} if exists $this->{l}{$key};

    # I don't know what you're doing, but eval is scary :)
    # Consider using a sub ref (sub { ... } or \&subname) instead
    $this->{l}{$key} = eval $this{f}{$key}; 

    # Create a new hash ref if the eval returned undef
    $this->{l}{$key} = { } if not defined $this->{l}{$key};

    return $this->{l}{$key};
}
[download]

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