As turnstep already pointed out QUERY_STRING is just an environment variable.

bash $ ( export QUERY_STRING="foo=bar&baz=foo+bar"; perl x.pl)
tcsh $ (set QUERY_STRING="foo=bar&baz=foo+bar"; perl x.pl)
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to see a list of all Environment variables available to your script you might want to try the following in as a cgi-script:

#!/usr/bin/perl
$\="\n"; $,="=";
print "Content-Type: text/plain\n";
map{print $_, $ENV{$_} } keys %ENV;
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