You could prevent the warning by testing the value before trying to use it like this

my $qs = $ENV{'QUERY_STRING'};
print NEW "$qs\n" if defined $qs and not $s='';
[download]

However, when $ENV{'QUERY_STRING'} is undefined or has a null value, it normally means you need to take some other action in your script to correct that situation.

Thats what makes the warning so useful.