This design document proposes that the "Fat Arrow" => operator will be changed in TT3 to do (more-or-less) exactly the same thing that it does in Perl: to quote the word on the left.

TT2 does some magical handling of arguments passed to a subroutine or method call. All named parameters are collected in a hash array and passed as the last argument.

[% foo(10, 20, x=50, 30, 40, y=60) %]
[% foo(10, 20, x=>50, 30, 40, y=>60) %]   # same as above in TT2

The foo() subroutine receives the arguments as if they were written:

[% foo(10, 20, 30, 40, { x => 50, y => 60 }) %]

The problem is that the subroutine you call might not be expecting this. If you don't want the magical behaviour then you must explicitly pass all arguments as positional values.

[% bar('x', 10, 'y', 20) %]

sub bar {
    my %points = @_;
    # ...etc...
}

TT3 will continue to do it's magical thing when the = operator is used. However, when => is used it will now do the same thing that Perl does - to automagically quote a bareword on the left.

[% bar(x => 10, y => 20) %]     # TT3's => works like Perl so
[% bar('x', 10, 'y', 20) %]     # it's equivalent to this

You can mix and match the two if you're really sadistic.

[% mad(10, x => 20, plink = 50, 30, y => 40, plonk = 60) %]

That's the same as this:

[% mad(10, 'x', 20, 30, 'y', 40, { plink => 50, plonk => 60 }) %]

The => operator is a useful idiom in Perl. It confuses Perl programmers that it doesn't work the way that it does in Perl.

You will no longer be able to treat => as a simple alias for = in an argument list. Hash definitions will continue to treat them both the same (as it does in TT2).

TT3 Language Design Document 4 - Blessing Named Parameters