From @MasterDuke17

perl6 -e 'sub foo($a where {* < 5}) { dd $a }; foo(3);'
===SORRY!=== Error while compiling -e
Malformed double closure; WhateverCode is already a closure without
curlies, so either remove the curlies or use valid parameter syntax instead
of *
at -e​:1
------> sub foo($a where {* < 5}⏏) { dd $a }; foo(3);
  expecting any of​:
  horizontal whitespace
  statement end
  statement modifier
  statement modifier loop

but

perl6 -e 'sub foo($a where {* < 5 and * > 9}) { dd $a }; foo(3);'
Int $a = 3

There's no error and the (impossible) where clause is ignored.

From @MasterDuke17

On Sat Oct 01 07​:02​:34 2016, ddgreen@​gmail.com wrote​:

perl6 -e 'sub foo($a where {* < 5}) { dd $a }; foo(3);'
===SORRY!=== Error while compiling -e
Malformed double closure; WhateverCode is already a closure without
curlies, so either remove the curlies or use valid parameter syntax instead
of *
at -e​:1
------> sub foo($a where {* < 5}⏏) { dd $a }; foo(3);
expecting any of​:
horizontal whitespace
statement end
statement modifier
statement modifier loop

but

perl6 -e 'sub foo($a where {* < 5 and * > 9}) { dd $a }; foo(3);'
Int $a = 3

There's no error and the (impossible) where clause is ignored.

It's not just where there are multiple *s, it seems to be whenever there's more than one expression

perl6 -e 'sub foo($a where {* < 5 or $a > 9}) { dd $a }; foo(6);'
Int $a = 6

From @MasterDuke17

Fixed in rakudo/rakudo#893

Tests added in Raku/roast@82115a385c

@MasterDuke17 - Status changed from 'new' to 'resolved'