I think the most cross-platform portable way to do this is polling, but glob and stat are actually pretty quick:

sub getNewestFile {
   my $dir=shift;
   my @files=glob "$dir/*";
   my %mtimes;
   map { $mtimes{$_}=(stat $_)[9] } @files;
   @files=sort { $mtimes{$b}<=>$mtimes{$a} } keys %mtimes;
   $files[0];
}
[download]

See what you think, burden on your system should be minimal if you do it less than once every second.

HTH,
SSF