Aside from the aesthetic aspect there is practically no problem with loops that only loop once or twice.

To shorten later access you could change the loops to

foreach my $host (keys %rez) {
foreach my $tgtp ( values %{ $rez{$host}{'tgt'} } ) {
foreach my $dev ( keys %$tgtp  ) {
 #do something with $tgtp->{$dev}
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You could also use map instead of foreach loops, but that would be just window dressing as map is just another type of loop