Why pass by reference?

[John M. Dlugosz]

In Perl 6, the default parameter passing is to make a read-only alias 
for the caller's lvalue.  This means that the function may not change 
the caller's variable, but must track changes to it made by other means.

What is the point?
It is a contrivance to illustrate how the variable can be changed by 
other means, and requires a global variable, the same variable passed as 
two different parameters, or the variable and a closure that affects the 
variable be passed.

In fact, this effect seems like something that should be warned against, 
not something that is touted as a feature.

It complicates the passing, requiring a read-only proxy or equivalent be 
introduced, making the readonly parameter more complex than the rw 
parameter.  It makes the actual access more complex, having to go 
through this extra layer.  It prevents optimizations, since you have 
more plumbing to go through and you have to watch for aliasing (that's 
the feature!) instead of assuming the value doesn't change between accesses.

In Perl 5, the @_ is a normal alias (read/write), and most parameters 
are copied to local variables, making it essentially pass-by-value.  The 
local variable is pass-by-value, and looking back at the @_ is pass by 
reference.  There is no feature like Perl 6's default readonly passing 
(read-only reference).  So it's not for historical use.  At least not as 
the default method!

So, I ask:  is there any reason to want this read-only reference as a 
passing method?  And if so, why does that preclude having the simple 
pass-by-value method available also?

From the typical Perl 5 usage, I would think that pass-by-value should 
be the default.

--John