...
my $q=new CGI;
my ($index);
print $q->header;
print $q->start_html(-title=>"Modif",
		     -author=>"cmic" );
print $q->start_form;
print $q->submit(-name=>"submit1", 
		 -label=>"submit1") ;
#
if ($q->param('submit1') eq 'submit1') {
    $index="Some val";
    print "DEBUG after 1st submit " . $index . "<br>";
    print $q->submit(-name=>"submit2", 
		 -label=>"submit2") . " ",
}
elsif ($q->param('submit2') eq 'submit2') {
    print "DEBUG after 2nd submit " . $index . "<br>" ;
}
print $q->end_form;
print $q->end_html;
...