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'});
}
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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.