crontab invokes a shell from your home directory with an arg0 of sh. If you need to set up your environment then run the .profile (or whatever your particular shell uses) as part of the cron statement.
I run the following from cron to successfully talk to an Informix database via DBI -

#Informix server processes
0 7 * * 1-5 . /.profile;. /usr/local/lib/dir_locations;. $INFORMIXDIR/
+lib/environ.ksh;$SERVERDIR/sbin/main_server.pl > /dev/null 2>&1
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The .profile sets my environment.
The file dir_locations sets common directories for all users such as INFORMIXDIR, extensions to $PATH.
The file environ.ksh sets the required informix settings
main_server.pl is the perl script that starts the whole ball rolling.

Hope that is of some help

Graham