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$ perl -le 'print ref \substr "foo", 1'
LVALUE

$ perl -le 'my $u; sub u :lvalue {$u} print ref \u'
SCALAR

#!/usr/bin/perl -l

...
print $u;

__END__

(append $u)='Bar';
print $u;

print +(append 'Foo')='Bar';

sub append :lvalue {
    local *LVALUE::STORE = sub {
...
    }
    $_[0];
}