$item is always an alias. In the 'failing' foreach, it's an alias to a constant in a list and hence not modifiable (where would perl save the modification?) For the 'working' foreach, $item is an alias to an array element which is modifiable (since we have some place to save the modification). So either use your array or make a copy of the aliased item:

(my $val = $item) =~ s/abc/zzzzzz/;
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