From @sorear

15​:14 < sorear> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar"
15​:14 <+p6eval> rakudo e393c7​: OUTPUT«Undefined value shifted from empty
  array␤current instr.​: 'perl6;Perl6Exception;throw' pc 14877
  (src/builtins/Seq.pir​:28)␤»
15​:20 <@​jnthn> I think we probably should just complain at parse time actually.
15​:20 <@​jnthn> We parse a signature there, but if you're starting to stick in
  optional parameters and stuff, we just say "oh noes, that's too
  complex to turn into a list".
15​:20 <@​jnthn> If you want full signature semantics, use binding, not
  assignment.
15​:21 <@​jnthn> Us not complaining about the signature being too complex is a
  rakudo fail.
15​:21 <@​jnthn> It's a bit annoying implementation wise.
15​:21 <@​jnthn> Because you parse a signature always but in the = case you want
  to do list assignment.
15​:22 <@​jnthn> So you need to turn the signature into a list.
15​:22 <@​jnthn> Which doesn't make sense if it's anything complex.

From @moritz

19​:36 <@​moritz_> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar"
19​:36 <+p6eval> rakudo 928f9d​: OUTPUT«a Any()␤»

Much better now, though not perfect.

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

From @coke

On Sat Jul 24 10​:38​:58 2010, moritz wrote​:

19​:36 <@​moritz_> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar"
19​:36 <+p6eval> rakudo 928f9d​: OUTPUT«a Any()␤»

Much better now, though not perfect.

Slight change​:

09​:20 < [Coke]> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar" #RT
  #​74664
09​:20 <+p6eval> rakudo ce5cca​: OUTPUT«a ␤»

--
Will "Coke" Coleda

From @MasterDuke17

On Wed Feb 01 06​:21​:05 2012, coke wrote​:

On Sat Jul 24 10​:38​:58 2010, moritz wrote​:

19​:36 <@​moritz_> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar"
19​:36 <+p6eval> rakudo 928f9d​: OUTPUT«a Any()␤»

Much better now, though not perfect.

Slight change​:

09​:20 < [Coke]> rakudo​: my ($foo, $bar = "baz") = <a>; say "$foo $bar" #RT
#​74664
09​:20 <+p6eval> rakudo ce5cca​: OUTPUT«a ␤»

perl6 -e 'my ($foo, $bar = "baz") = <a>; say "$foo $bar"'
Use of uninitialized value $bar of type Any in string context.
Methods .^name, .perl, .gist, or .say can be used to stringify it to something meaningful.
  in block <unit> at -e line 1
a

perl6 --version
This is Rakudo version 2016.09 built on MoarVM version 2016.09
implementing Perl 6.c.

If I get this right, this should complain about the fact that it's a list. Saying that it's uninitialized anyway makes no sense... Also, same problem in 2020.05

Updated:

my ($foo, $bar = "baz") = <a>;
say ~$foo;
say ~$bar; # Use of uninitialized value $bar of type Any in string context.

Not totally sure this is incorrect now. Since the pair is assigned a list with a single element, the non-existing second element clobbers what's in there.