Awhile back we had an issue where a coworker wanted to use one of my Perl CGI scripts inside of her scripts, using the backticks method to have it spit its output to the webpage. But for some reason, my script refuse to recognize the arguments she was passing it. The weird thing was, if we ran her script from the command line, everything worked fine.

The issue is in the way CGI.pm detects whether it is being called from a web browser or from the command line. It checks for the presence of the REQUEST_METHOD environmental variable. If it is present, CGI knows to get its arguments from the web browser.

When the inner script it called either using system() or the backticks method, it inherits the REQUEST_METHOD environmental variable from the calling program! It therefore thinks it’s being called directly from Apache instead of the command-line shell, and ignores the arguments. The trick to get it working is to temporarily clear the REQUEST_METHOD environmental variable:

my $cmd = "./gen_report.pl search_date=10/2009 pdf=1";

#
# Call gen_report.pl script to generate the report.  Since gen_report.pl is
# a CGI script that we are calling from the command line, we need to clear
# out the request method variable for CGI.pm
#
my $request_method = $ENV{ REQUEST_METHOD };
$ENV{ REQUEST_METHOD } = '';
my $display = `$cmd`;
$ENV{ REQUEST_METHOD } = $request_method;