For substitution, you can use ~s///. This will just substitute the file date with your date. Also the result has a date 082411. I assume that $nDateStamp holds this value. And $tDateStamp holds 082511.

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

$filename = '17000_DM161_082511.dat';

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

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

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