How to do a substring replacement in Perl 6

[Douglas E. Miles]

Hi all!

From http://perldoc.perl.org/functions/substr.html:
substr has a 4th argument (or you can use substr as an lvalue) for 
replacement.
substr EXPR,OFFSET,LENGTH,REPLACEMENT

From http://doc.perl6.org/routine/substr I don't see any way to do a 
replacement. What is the idiomatic way of doing a positional string 
replacement in Perl 6? Thanks!