#keep based on pattern match
opendir(DIR, $destinationDirectory) or die $!;
while(my $file = readdir(DIR)){
  next if -d $file;#skip directory entries
  unlink unless $file =~ /$pattern/;
}

##</code><code>##

#keep based on a prepopulated hash
%prepopulatedHash =(FILE1 => 1, FILE2 => 1, FILEN => 1);
opendir(DIR, $destinationDirectory) or die $!;
while(my $file = readdir(DIR)){
  next if -d $file;#skip directory entries
  unlink unless exists($prepopulatedKeepHash{$file};
}