when used in scalar context, readdir returns undef when there are no more entries:

opendir DIR, q{.} or die $!,$/;
my $last;
while (my $file = readdir DIR) {
   $last = $file;
}
print $last; ## this prints the last file
closedir DIR;
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yet, keep in mind that there is no order garanteed, so don't count on that.

OTOH, if you use readdir in list context, it will return you a null list when no entries are available.

opendir DIR, q{.} or die $!,$/;
my @files = readdir DIR;
closedir DIR;
print $files[-1];  ## this prints the last file
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Here you have control on the file ordering in the usual list sorting manner:

## sorted by name, case sensitive
my @files = sort {$a cmp $b} readdir DIR;
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