Under ordinary circumstances you never need to "use DBD::Foo", you simply "use DBI" then name Foo in the DBI connect string e.g.

use DBI;
my $dbh = DBI->connect('dbi:Informix:...
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The errors you're getting look like you may have an improperly installed DBD::Informix but I'd suggest trying it with the proper connection string first and reinstall if you get similar errors.