Your second "anomaly" has to do with foreach loop aliasing. Particularly:

If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over.

So:

my @a = (1,2,3,4);
for my $i (@a) { $i = 5 };
print "@a";  # prints: 5 5 5 5
[download]

while:

for my $i (1,2,3,4) { $i = 5 };
[download]

errors out with: Modification of a read-only value attempted ...

I think what's happening with your 3 examples of the second anomaly is this:

1. foreach my $letter ('a' .. 'c', 'd' .. 'f') creates a list of lvalues due to the comma operator in the middle. Perhaps this is done only once at compile time? which explains the persistence.
   
2. foreach my $letter ('a' .. 'f') has been optimized as a counting loop, so the range operator does NOT generate a list.
   
3. foreach my $letter (@letters) always has @letters redefined right before the loop, so each call to letters() yields consistent results.
   

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