I'm guessing (because it doesn't look like you posted a fully functional example), but -- if you intend $1to be the first parameter sent to the subroutine which contains the code you posted, you need to grab that in advance:

sub doMyThing
{
    my ($changeNumber, @otherParameters) = @_;
       :
       :
    print $q->start_form(
        -name    => 'main_form2',
        -method  => 'GET',
        -action  => "http://xxx.com/cgi-bin/perforce?Submit=describe\&
+changenum=$changeNumber",
        -target  => '_blank',
    );
       :
       :
    return;
}
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