open OLD, $oldfile or die;
open NEW, "> $oldfile.new" or die;
my $flag = 0;
while (<OLD>) {
 $flag =1 if /pattern/;
 print NEW unless $flag;
 $flag = 0 if /end_pattern/;
}
rename ("$oldfile", "$oldfile.old") or die;
rename ("$oldfile.new", $oldfile) or die;