Hm, good question: how for a script to determine how it was invoked: command line vs. crontab.

The only thing I can offer is that when run from crond, different environment variables are set. The one that seems to be helpful is $TERM. When a script is run from crond, at least on my Redhat 6.x system, TERM is set to "dumb".

So maybe something like:

if ($ENV{TERM} eq 'dumb') {
  ## redirect stdout to stderr
  open STDOUT,">&STDERR";   ## I think :)
}
print "This will go to stderr if running from crond\n";
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