Aha. That's it. The explicit return seems to make the sub return the value of the rhs, but the variable is never modified. That's what "experimental" means.

This works as expected:

#!/usr/bin/perl -wl
use strict;

{
    my $count = 0;
    sub counter : lvalue { ++$count }
}

print counter() for 1..5;
print counter() = -5;
print counter() for 1..5;
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