Problem with perl check

Using nagios version 1.05b with a custom check_snmp perl program (embedded
perl is not being used).

Running it from the command line, it correctly returns 0 for OK and outputs
the appropriate values (single line of output, approximately 25 characters).
However, when run as a service check via Nagios, it returns a value of
UNKNOWN (the returned data states no response from host).  

Note that the command line check of the script was executed from the same
machine running Nagios, using the same options as the nagios service check.

Any ideas why the machine Nagios is running on can contact the device, but
nagios cannot?

Mike