Yes, do not attempt to read from $ENV{'QUERY_STRING'} if it is not defined. Change that line to:

if (defined $ENV{'QUERY_STRING'}) {
    # assign to variable
    # check for login parameter
}
[download]

I'd like to echo davorg's update, though, and suggest you use CGI.pm instead:

use CGI;
my $q = CGI->new();

my $age = $q->param('age');
my $login = $q->param('login');

if ($age) {
    # do whatever
}

if ($login) {
    # do whatever
}
[download]

With a mighty WHOMP!, these visible warnings (and several invisible problems probably lurking in your CGI parser) will automagically disappear.