I don't understand, what you call 'not an array reference' is what I call 'object', so don't we mean the same thing?

In:

use ar ;
my @ar ;
my $art = tie @ar, "ar" ;
@ar = (1, 2) ;
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My point is, I cannot know what $art is, so to me this is a big no no:

   my @ar-deref = @{$art};
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Agree. But the following is perfectly ok IMHO

for(@ar) {
  print "$_\n";
}
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Yes, that is perfectly fine IMO too

My point is, I cannot know what $art is, so to me this is a big no no: my @ar-deref = @{$art};

Yes, I agree that is bad (unless $art overloads @{}), but in the code you showed here, for (tie @ary, "My::Class"), there is no such "bad" de-reference of the return value of tie happening, not even an implicit one. for (tie @ary, "My::Class") { print "$x\n" } will just print something like My::Class=HASH(0x...) - as perlancar said, it's just looping over a one-element list, the return value of tie, and not the elements of @{$art}, as you seemed to be worried about.

I absolutely agree with that too. I just interpreted a different intent:

for (tie @ary, "My::Class", "some", "el", "ems") { ... } # NOPE
# This makes for() only loops over a single value, the tied object.
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If the intention was to iterate over the 'object' then a possible solution would be to use a de-reference. However I felt this would be a bad idea and decided to point this out.

But it doesn't matter, everyone has proposed solutions that are much better. Esp. the solution hdb has found now using the lvalue sub is absolutely fantastic.