> Do you see any problems with the suggested solutions?

Yes, if a value is undef I want to see undef in the debug output, not an empty string.

And I want to be able to distinguish the string "undef" from undef.

Consequently you should use Data::Dump which already handles those cases and more

#extended usage with names
 print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);
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of course that's cumbersome, so you want to put it in a custom debug routine, like your hash syntax

But of course you might not really want to repeat the var name.

Then you can use an approach like

Data::Dumper::Lazy

use Data::Dumper::Lazy;
@a = 1..5; 
dmp {@a};
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Contrary to other solutions relying on source filter or Padwalker this will also work within string eval.