A) http://www.host.com/cgi-bin/test-script.pl
B) http://www.host.com/cgi-bin/test-script.pl?param=value
C) http://www.host.com/cgi-bin/test-script.pl?param=value&color=blue

 1: #!/usr/bin/perl -w
 2:
 3: warn $ENV{'QUERY_STRING'};

Line 3 returns:

A) blank (null string, no data)
B) param=value
C) param=value&color=blue