nnested subs provide some more abstraction ability

Not in Perl.

Whether nested or not, the following two snippets are equivalent:^[1]

sub foo { ... }
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BEGIN { *foo = sub { ... }; }
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As such,

• Inner sub aren't scoped to outer sub.
• It misleadingly appears the inner sub is scoped to the outer sub when they're not.
• The inner sub can't use lexicals of the outer sub.
• If the inner sub attempts to use lexicals of the outer sub, you get very weird and useless behaviour.
• If the inner sub attempts to use lexicals of the outer sub, you get a warning.

As you can see, you gain no abstraction or other benefits. Everything about it is bad.

That said, none of this applies to lexical inner subs. This is fine:

sub outer {
   ...

   my sub inner {
      ...
   }

   ...
}
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---

1. Exception: The first has a name attached to the sub itself, while the second doesn't. This difference will affect croak and strack traces. I could have used Sub::Util's set_subname to correct this, but loading the module would introduce a new difference.