From @masak

Basically, any argument containing only whitespace seems to trigger
this error. It seems that the presence of the MAIN sub is part of the
error; remove it and it accepts all of the inputs causing an error
below.

$ cat x.pl
sub MAIN {}

$ perl6 x.pl -y=' '
Cannot invoke this object
  in block <unit> at x.pl​:1

$ perl6 x.pl -y=''
Cannot invoke this object
  in block <unit> at x.pl​:1

$ perl6 x.pl -y=`perl -E'say "\n"'`
Cannot invoke this object
  in block <unit> at x.pl​:1

$ perl6 x.pl -y=`perl -E'say "\t"'`
Cannot invoke this object
  in block <unit> at x.pl​:1

$ perl6 x.pl -y=`perl -E'say "\t "'`
Cannot invoke this object
  in block <unit> at x.pl​:1

$ perl6 --version
This is perl6 version 2015.10-89-g36c4c6a built on MoarVM version
2015.10-14-g5ff3001

From @skids

The examples would fail usage constraints with the parameterless MAIN.
I could not get them to fail with any internal messages on current rakudo,
either with the parameterless MAIN or with a MAIN that had a :$y.

Haven't looked as to when this got fixed.

Tests added for both cases (though I had to add a sub USAGE when testing
the parameterless case) in Raku/roast@bdf67b698b,
so closing as resolved.

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

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