You're on the right track. How about this?

chdir("c:/tdt/src") || die "trying"; # Or whatever works

foreach my $tdt (<*.tdt>)
{
     system("$BASEDIR/nbsstools/tdtgen/bin/tdtgen", $tdt);
}
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This uses the glob feature. By the way, make sure your chdir call succeeds.

As a note, your test /\.tdt/ is equivalent to *.tdt* which means that foo.tdt.txt will also match.