Define a variable in your crontab, which can be detected by the script: 
CRON = 1
* * * * * scriptname

[download]
Check for it in the script and direct output accordingly: 
BEGIN
{
    open (STDOUT, ">&STDERR") if ($ENV{'CRON'});
}

[download]
This will allow you to print normally, but it will be directed to STDERR using a redirect. The Camel Book has more detail on the "&" operator in the context of an open request, as does "perldoc -f open".

Note: chipmunk's reply below had a better (i.e. working) solution which I have incorporated here to avoid giving out bad (i.e. broken) advice.

Previously I suggested checking for the environment variable '_', which seemed to only appear in cron, but is in fact, an artifact of the 'sh' shell and derivatives, such as bash. I used 'tcsh' for interactive, and it does not have that variable defined, hence the confusion.

In reply to Re: Detecting running from cron? by tadman
in thread Detecting running from cron? by oakley

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