s:g/T/U/ doesn't work ?

[Marc Chantreux]

hello perl6 people,

On

    This is perl6 version 2012.09.1
    built on parrot 4.6.0 revision 0

When i try to run

    use v6;
    use Test;
    for 'GATGGAACTTGACTACGTAAATT' {
        s:g/T/U/;
        is $_
        , 'GAUGGAACUUGACUACGUAAAUU'
        , 'RNA';
    }

I get

    Cannot assign to a non-container
      in sub infix:<=> at src/gen/CORE.setting:11692
      in block at /tmp/ZZZ:4

As far as i understand from the doc, s:g/T/U/ is a valid syntax. any idea? 

regards
marc