Why is this Perl 6 feed operator a "bogus statement"?

I took this example from Day 10 – Feed operators of the Perl 6 2010 Advent Calendar with the slight change of .uc for the .ucfirst that's no longer there:

my @rakudo-people = <scott patrick carl moritz jonathan jerry stephen>;
@rakudo-people
    ==> grep { /at/ } ==> map { .uc } ==> my @who-it's-at;
say ~@who-it's-at;

I write it slightly differently with some additional whitespace:

my @rakudo-people = <scott patrick carl moritz jonathan jerry stephen>;
@rakudo-people
    ==> grep { /at/ }
    ==> map { .uc } ==> my @who-it's-at;
say ~@who-it's-at;

Now it's a "bogus statement":

===SORRY!=== Error while compiling ...
Bogus statement
------>         ==> grep { /at/ }⏏<EOL>
expecting any of:
    postfix
    prefix
    statement end
    term

This isn't a problem with just this example. Some examples in the current docs can exhibit the same behavior.

If I add an unspace to the end of the offending line, it works again:

my @rakudo-people = <scott patrick carl moritz jonathan jerry stephen>;
@rakudo-people
    ==> grep { /at/ } \
    ==> map { .uc } ==> my @who-it's-at;
say ~@who-it's-at;

Curiously, a comment at the end of that line does not work. I would have thought it would have eaten the offending whitespace.

The feed operator says:

In the case of routines/methods that take a single argument or where the first argument is a block, it's often required that you call with parentheses

That works:

my @rakudo-people = <scott patrick carl moritz jonathan jerry stephen>;
@rakudo-people
    ==> grep( { /at/ } )
    ==> map { .uc } ==> my @who-it's-at;
say ~@who-it's-at;

But why wasn't that a problem in the first form? What is the whitespace doing here? And what situations are included in "often required"?