This is possible in pure perl, if you can live with using a global var?

$buffer = 'x' x 10000000;
# memory used 12,200kb

*x = \$buffer;
# mem used 12.200kb

print length $x;
10000000

print substr $x, 0, 30;
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[download]

$x is now an alias for the buffer and can be used as any scalar can be. However, I don't think that there is any way to use a glob assignment with a hash element.

A possible alternative would be to wrap an lvalue sub (or method) around the $buffer.

sub test : lvalue { $buffer };

print length test;
10000000

print substr test, 0, 10;
xxxxxxxxxx

substr( test, 3, 3 ) = 'ABC';

print substr test, 0, 10;
xxxABCxxxx

print length test;
10000000
[download]

This behaves like a scalar for most purposes albeit that the syntax looks a little strange. As a method, the syntax would be

 
print substr obj->test, 20_000, 5;
substr( obj->test, 20_000, 5 ) = 'hello';
[download]

which isn't too aweful. It would mean utilising a global with the risk of being stomped on elsewhere, but using a suitably obscure name should minimise the risk of that.