DBD::Informix -1820

I'm using DBD::Informix and mod_perl and was having a problem with a 
-1820 error (-1820: Host variable type has been changed between fetches 
or puts) hitting a particular query on a second or subsequent execute of 
a prepared statement.

So I decided to check the DBD build and running the make test I get

t/t93lvarchar.t ..... DBD::Informix::st fetchrow_hashref failed: SQL: 
-1820: Host variable type has been changed between fetches or puts. at 
/root/.cpan/build/DBD-Informix-2008.0513-jLu21N/blib/lib/DBD/Informix/TestHarness.pm 
line 669

Which is what I'm seeing with a fetchrow_array under mod perl

IBM Informix CSDK Version 3.50, IBM Informix-ESQL Version 3.50.UC8
IBM Informix Dynamic Server Version 11.70.FC1IE
This is perl, v5.8.8 built for i386-linux-thread-multi
Client is on Centos 5.2

under mod_perl I can make the problem 'go away' by re-preparing before 
each execute, but that seems to be sub-optimal :-)

Anyone seen anything similar?

TIA

--
Clive