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
[download]

Prints:

TDATE 17/11/10 SDATE 22/11/10
[download]

perlre