use strict;
opendir(DIR,$newfile) or die "Blerch: $!\n";
my %files=map {$_,1} readdir(DIR);
#assume %oldfiles was set before in a similar way
foreach my $file (keys %newfiles) {
  unless (exists $oldfiles{$file}) do_we_have_a_new_file(); #Cheers and happiness!!
}