From @peschwa

14​:20 masak m​: say { :a(2) :b(3) :c(4) }.perl
14​:20 camelia rakudo-moar 7dd729​: OUTPUT«{"a" => 2}␤»
14​:20 masak o.O
14​:21 I postulate that this is wrong and that the output hash should have three items.
14​:21 any second opinion?
14​:21 psch masak​: i think that's already ticketed somewhere
14​:21 i remember asking about something similar some time back

Alas, I was wrong, it's apparently not yet ticketed. I'm unable to find a reference in the spec that says it should work, but a similar issue had been raised previously in [1] and #perl6 thought it was valid then and thinks it is valid still.

[1] http://irclog.perlgeek.de/perl6/2013-12-22#i_8031696

From @FROGGS

That's my first attempt at solving this issue​: https://gist.github.com/FROGGS/199ccc0d6412e6858217

TL;DR​:
$ perl6-m -e 'say( (​:a(2) :b(3) :c(4)) )'
"a" => 2 "b" => Mu "c" => Mu

The RT System itself - Status changed from 'new' to 'open'

From @FROGGS

Patch for (​:a(1) :b(2))​: rakudo/rakudo@55d59e461a
Patch for {​:a(1) :b(2)}​: rakudo/rakudo@c40c1b861f
Tests​: https://rt-archive.perl.org/perl6//Public/Bug/Display.html?id=123215

@FROGGS - Status changed from 'open' to 'resolved'