I assume that $1 is the result of a regular expression match. If not you should change it to a textual name. The reason the target url is not recognising the query parameters is that this is a POST request. You need to add some hidden fields to pass those parameters:

my $changenum = $1;
print $q->start_form(
    -name    => 'main_form2',
    -method  => 'POST',
    -action  => 'http://xxx.com/cgi-bin/perforce',
    -target  => '_blank',
);
print $q->hidden(
    -name     => 'Submit',
    -value    => 'describe',
);
print $q->hidden(
    -name     => 'changenum',
    -value    => $changenum,
);
print $q->submit(
    -name     => 'submit_form2',
    -value    => 'See the change',
);
print $q->end_form;
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