$nDateStamp = '082411';
$tDateStamp = '082511';

$filename = '17000_DM161_082511.dat';

if (index($filename,$tDateStamp) != -1) {
  $filename =~ s/$tDateStamp/$nDateStamp/;
  print "found\n$filename\n";
}

##</code><code>##

$nDateStamp = '082411';
$tDateStamp = '082511';

$filename = '17000_DM161_082511.dat';
if ( $filename =~ s/$tDateStamp/$nDateStamp/) {
    print "found\n$filename\n";
}