Use a variable and assign an expression to it in one step in Raku

You mean, something like this?

my @l = <a b c d e f g h i j k>; 
say @l[ 0, (* + 1) * 2 ...^ * > 7 ]; # says a c g;

A little bit more verbose:

my @l = <a b c d e f g h i j k>; 
say @l[ 0, -> $i { ($i + 1) * 2 } ...^ -> $i { $i > 7 } ];

Or even

my sub next-i( $i ) { ($i + 1) * 2 };
my sub last-i( $i ) { $i > 7 };

my @l = <a b c d e f g h i j k>; 
say @l[ 0, &next-i ...^ &last-i ];

Edit: Or, if as in the comment below you know the number of elements beforehand, you can get rid of the end block and (simplify?) to

say @l[ (0, (* + 1) * 2 ... *)[^3] ];

Edit:

using a variable and assigning an expression to it in one step

Well, the result of an assignment is the assigned value, if that is what you mean/want, so if you insist on using a while loop, this might work for you.

my @l = <a b c d e f g h i j k>; 
my $i = -1; say @l[ $i = ($i + 1) * 2 ] while $i < 3;

my @l=<a b c d e f g h i j k>;
my $i=0;
say @l[($=$i,$i=($i+1)*2)[0]] while $i < 7'

a
c
g

A bit of cheating using $. Otherwise, it didn't work...

I would have thought ($i,$i=($i+1)*2)[0] would have done the trick.