Things like this might strike you another couple of times. And basically the only thing I can say about this is RTFM. But that's too unfriendly. Besides, it is not helpful at all.

I am not familiar with the DBI modules at all, but as soon as I saw that you where trying to $scalar = Package->constructor or $scalar->method I realised what the cause of the error must've been.

The funny thing is that, in a way, you noticed it yourself too.

But I could have sworn I just defined $dbh. What gives?

Well, did you define it? "Yes," you say, "look: $dbh = DBI->connect(...)"

Agreed, that looks like you're defining something. Fortunately you're aware some error might occur and you're checking for that...

... only to get Perl to complain! Undefined value, wtf?

Now, this is the time to look at how Perl modules and simple functions often work: on success they return something true (in this case a DBI object). On failure they return something false, mostly undef.

That is a nifty feature that allows for structures like

if (doSomething) {
    print "it worked";
} else {
    print "oh no");
}
[download]

That, you know. Or at least you use it: $dbh = DBI->connect(...) or die(...);

Well. To get the code after or to be executed, the code before or must evaluate to a false value. I guess you know that too.

If DBI->connect simply succeeds, it just returns an object. That can never be false.

But if we ever want to get to the part after the or, then DBI->connect must return a false value. By definition, this can never be an object. So the $dbh->errstr can never work there.

I started this reply with stating that you should peek into the manual. Have a look at the documention DBI's connect method.

If the connect fails (see below), it returns undef and sets both $DBI::err and $DBI::errstr. (It does not explicitly set $!.) You should generally test the return status of connect and print $DBI::errstr if it has failed.

Besides that the direct answer to your question is just there, it also explicitely states that connect returns undef on failure - you already knew that, as I pointed out - and undef of course never has any methods on it.

Keep that in mind: if you can do $scalar = something() or doSomethingElse(), then generally $scalar will not hold the value it would've had if something was just succesful.

I'm sorry if I sounded somewhat patronizing. I'm also sorry if I offended anyone, especially you, punch_card_don, when I said you just need to RTFM. So far for my futile attempt at a disclaimer.