If a DBI->connect fails, there will be an error string set in DBI::errstr. This is analogous to the $dbh->errstr when you have an open db handle. The perldoc on DBI will have more info.

e.g.

my $dbh = DBI->connect('DBI:stack_of_papers_on_my_desk', 'user', 'pw')
    or die DBI::errstr;
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