I think I've struck it before ... something to do with the wonderful Copy-On-Write, IIRC.

Quite the opposite. If anything, it's about the lack of copy on write.

$x and $y are references to objects. You are copying the reference, but not the referenced object (on write or otherwise).

You want $y = $x->copy(); to create a clone.

Is there some way (apart from building perl without COW) to guard against getting bitten by this ?

To avoid getting bit by this, only uses classes that provide immutable objects.

Also, you'll need to avoid references to arrays and hashes, as the same issue is found there. 

my $x = [ 4 ];
my $y = $x
++$_ for @x;
say @y;  # 5!!

[download]

Even then, you'll have to worry about aliases.

Is it a bug in perl ?

It's a performance concession. Using immutable objects has a high cost.

You brought up Perl's COW mechanism for strings. It's specifically a mechanism to mitigate these costs. For strings. Imagine having to build COW into all your classes... And how it would work for classes that reference external (to perl) resources?

In reply to Re: Action at a distance by ikegami
in thread Action at a distance by syphilis

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