while (<DATA>) {
    if (/(PURCHASED ON|SOLD ON|SETTLEMENT ON)/) {
        $desc = $_; 
        if ($desc =~ /SETTLEMENT\s+ON\s+(\S+)/) {
            $sdate = $1;	
        }
        if ($desc =~ /(?:PURCHASED|SOLD)\s+ON\s+(\S+)/) {
            $tdate = $1;	
        }
        print "TDATE $tdate SDATE $sdate\n";
    }  	
}

__DATA__
CLASS A SOLD ON 17/11/10 FOR SETTLEMENT ON 22/11/10 @ 145.59993

##</code><code>##

TDATE 17/11/10 SDATE 22/11/10