To me, this scenario isn't covered by either of the given explanations.

It's covered by the second one. %data has gone out of scope. The lexical block in which it resides (the file) was exited before AUTOLOAD was called.

A sub can extend the life of a lexical by becoming a closure and capturing the lexical. However, a sub only captures the variables it needs, meaning it only captures the lexicals it references. Other lexicals aren't captured.

If there's a reference to (i.e. a mention of) %data in AUTOLOAD (e.g. if you uncomment any of the commented statements), then the sub captures %data, and the eval will find %data.

If there are no references to %data in AUTOLOAD, then the sub doesn't capture %data, so it won't exist when eval is evaluated.

Adding «%data if 0;» to AUTOLOAD is enough to make it capture the lexical without warnings.

$ perl -wE'{ my $s=1; sub f {          eval q{$s} } } say f || 0;'
Variable "$s" is not available at (eval 1) line 2.
0

$ perl -wE'{ my $s=1; sub f { $s if 0; eval q{$s} } } say f || 0;'
1
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Another option is to use a package variable since they don't go out of scope.

$ perl -wE'{ my  $s=1; sub f { eval q{$s} } } say f || 0;'
Variable "$s" is not available at (eval 1) line 2.
0

$ perl -wE'{ our $s=1; sub f { eval q{$s} } } say f || 0;'
1
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Then, running perl Foo.pm works fine

I cringe whenever someone does

perl -c Foo.pm
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perl -e'use Foo;'
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because they're not equivalent. The crucial difference in this case is that you now call AUTOLOAD before the lexical scope of %data is exited.