I want to add that if you were checked what returns DBI->connect then the reason of this problem would be obvious to you. Something like:

my $dbh = DBI->connect(....) or die $DBI::errstr;
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Another option for handling of DBI errors is usage of exceptions. DBI->connect has option RaiseError which can enable thowing exceptions if there exist any problem.

my $dbh = DBI->connect($dsn, $user, $password, { RaiseError => 1 });
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In former case you don't need to check what DBI->connect returns. Instead you should put this code inside eval block if you want to handle errors.