Thanks for all the replies. It seems that I cannot do this (easily) with Hook::LexWrap, and the best way is to wrap manually or use alternatives like Hook::WrapSub (at least in cases where one does not need lexical wrappings that unwrap themselves when falling out of scope).

But this is a bug in Hook::LexWrap, right? The documentation seems to indicate that it should be possible, and there is even example code illustrating this feature (which does not seem to work)

 wrap set_temp,
                pre   => sub { splice @_, 0, 1, $_[0] * 1.8 + 32 },
                post  => sub { $_[-1] = ($_[0] - 32) / 1.8 };
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