On Thu Aug 28 01:53:46 2003, vparseval wrote:
>     #! /usr/bin/perl -w
> 
>     package Tie::Prematch;
>     sub TIEHASH { bless \my $dummy => __PACKAGE__ }
>     sub FETCH   { return substr $_[1], 0, $-[0] }
> 
>     package main;
> 
>     tie my %pre, 'Tie::Prematch';
>     my $foo = "foobar";
>     $foo =~ s/.ob/$pre{ $foo }/;

The problem here is that the Perl compiler sees the RHS of the s/// as
being invariant and thinks it can get away with evaluating without
setting up the last-match context.

At the optree level the difference is the inclusion or otherwise of the
substcont OP code.

Since this is a compile time optimisation and the tied-ness of %pre
cannot be determined at compile time there's not much that can really be
done to fix this (other than ditch the optimisation completely - which
would be a very high price to pay).

Arguably this (c|sh)ould be documented in perlop and/or perltie.