Yes, I agree, I also would have expected a warning (which is why I added the -w flag in the first place).

Just as I would also have expected a warning saying something like "attempt to modify read-only value" with the following pieces of code:

$ perl -Mstrict -wE 'say ++$_ for 1..4;'
2
3
4
5

~
$  perl -Mstrict -wE 'say $_ for map {++$_} 1..4;'
2
3
4
5
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But there is no warning. On the other hand, you get a warning with this:

$ perl -Mstrict -wE 'say $_ for map {++$_} (1, 2, 3, 4);'
Modification of a read-only value attempted at -e line 1.
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There must be some reason, but it does not look very consistent to me.