If there is no mechanism presently, is this kind of thing worth implementing as a patch?

I don't know if there's an lvalue-sub way of doing that, but you could turn it all around, and not have a function that is an lvalue, but have an lvalue that is a couple of functions!

That's right, I'm talking about tie. 

package Tie::Fooz;
sub TIESCALAR { bless \(my $dummy), $_[0] }
sub FETCH { ${ $_[0] } }
sub STORE { ${ $_[0] } = $_[1] }

package main;
tie my $foo, 'Tie::Fooz';

[download]
Unfortunately, I have not a clue what your code should do or why you would want to use a $foo_new, so I couldn't create an equivalent tie. Anyway, STORE serves as your islvalue condition, and FETCH for not islvalue, I think. 

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In reply to Re: Side Context in an 'lvalue' Subroutine by Juerd
in thread Side Context in an 'lvalue' Subroutine by tadman

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