for my $source ( keys %dir )
{
 my $destination = $dir{$source};
 find( sub 
	{ 
  my $relativeName = $File::Find::name;
  $relativeName =~ s/$source//;
  my $sourceName = File::Spec->catdir( $source, $relativeName );
  my $destinationName = File::Spec->catdir( $destination,
  $relativeName );
(...compare and backup if new or newer ...)
}