$foundit = 0;
while (<$FH>) {
    $foundit = 1 if /^pattern/;
}

if ($foundit) {
    do some processing
}

##</code><code>##

open FH,"file" or die "Can't open file : $!\n";
$count1 = 0;
while (<$FH>) {

$count++ if /^pattern/;
}
close FH; 
open FH,"file" or die "Can't open file : $!\n";
$count2 = 0;
while (<$FH>) {
   if (/^pattern/ and $count2 < $count1) {
       $count2++;
   } else {
       do some processing
   }
}