You're going to have three loops no matter what. You seem to be asking how to hide them. That's what subs are for.

sub extract {
   my ($rez) = @_;
   my @flattened;
   for my $host ( keys %$rez ) {
   for my $tgt  ( keys %{ $rez->{$host}{tgt} } ) {
   for my $dev  ( keys %{ $rez->{$host}{tgt}{$tgt} } ) {
      push @rv, [ $tgt, $dev, $rez->{$host}{tgt}{$tgt}{$dev} ];
   }}}
   return @flattened;
}

for (extract(\%rez)) {
    my ($tgt, $dev, $val) = @$_;
    ...
}
[download]

Or a callback approach:

sub walk(&@) {
   my ($cb, $rez) = @_;
   for my $host ( keys %$rez ) {
   for my $tgt  ( keys %{ $rez->{$host}{tgt} } ) {
   for my $dev  ( keys %{ $rez->{$host}{tgt}{$tgt} } ) {
      $cb->($tgt, $dev, $rez->{$host}{tgt}{$tgt}{$dev});
   }}}
}

walk {
    my ($tgt, $dev, $val) = @_;
    ...
} \%rez;
[download]