I don't think that you can trap other constant modules with a block eval either. The issue lies with Perl, which doesn't allow a function/subroutine on the left-hand side of an assignment, unless that function is already known as assignable through the :lvalue attribute:

> perl -wle "sub foo {}; eval { foo()='baz' }"
Can't modify non-lvalue subroutine call in scalar assignment at -e lin
+e 1, near "'baz' }"
Execution of -e aborted due to compilation errors.
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Marking the subroutine with :lvalue lets Perl know that we intend to let the function appear on the left-hand side of an assignment:

> perl -wle "sub foo:lvalue {}; eval { foo()='baz' }"
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All "constant" modules are syntactic sugar for defining functions that take no parameters, because that's how sigil-less constants work in Perl.