It depends on your server environment... how are the CGIs being invoked?

You could check $ENV{GATEWAY_INTERFACE}, though it might be a mod_perl only thing. (haven't written straight CGI in a while...) This code is invoked before any of my mod_perl scripts are run.

# make sure we are in a sane environment.
$ENV{GATEWAY_INTERFACE} =~ /^CGI-Perl/ or die "GATEWAY_INTERFACE not P
+erl!";
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You could compare dumps of the %ENV hash, when invoked from the command line vs. CGI. Something like:

print "$_ => $ENV{$_}<BR>\n" for (keys %ENV);
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-Blake