my ($buf, $onelineback, $twolinesback, $thisline);
while($buf = <IN>) {
if ($buf =~ /somestring/) {
    if defined($twolinesback) {
       print $twolinesback;
    } else {
       print "Matched, but not enough lines read yet\n";
    }
}
$twolinesback = $onelineback;
$onelineback = $thisline;   
$thisline = $buf;

}