You'll have to show us the code.

$dbh = DBI->connect($data_source, $username, $password)
                       or die $DBI::errstr;
[download]

I agree with [id://Joost]. More than likely, ORACLE_HOME is set for your user environment but not for your cron. We have a standardized wrapper for cron jobs:

#!/bin/sh

#
# Generic cron job wrapper
#

# Modify recipient list as needed.

RECIPIENTS="root@foo.com"

CRON_OUTPUT=/tmp/cron.output.$$

(

###########################################################

# Insert cron job statements here
# PREFIX=/path/to/job
# ENV_VAR=what_you_need
# export ENV_VAR
# cd $PREFIX
# ./job

###########################################################

) > $CRON_OUTPUT 2>&1


if [ -s $CRON_OUTPUT ]
then
        (
        echo 'Your "cron" job on '`hostname`
        echo ""
        echo "$0"
        echo ""
        echo "produced the following output:"
        echo ""
        cat $CRON_OUTPUT

        ) | mail -s 'Output from "cron" command' $RECIPIENTS
fi

/bin/rm $CRON_OUTPUT
[download]

Maybe a bit overkill but by always using a wrapper, I never forget (well almost never) to set need env vars.
-derby

The key here is dbi:Oracle:db2 - you're using Oracle. The Oracle libraries need to have several environment variables set before they will load at all.

As I already mentioned in the chatterbox, several times, you want to load the environment variables, best before launching your Perl script. Either do this by writing a small shell script wrapper around your Perl script that sets up the Oracle environment or set up the environment manually by setting the values in %ENV from within Perl, or use, as I pointed out, Get Default Environment or a variation thereof to load the environment values set up by some shell script. A module similar to that node is Shell::EnvImporter.

Phil