G'day Rolf,

$ perl -v | head -2 | tail -1
This is perl 5, version 18, subversion 2 (v5.18.2) built for x86_64-li
+nux-thread-multi
$ alias perle
alias perle='perl -Mstrict -Mwarnings -Mautodie=:all -E'
[download]

I first tried a cut-down version of your issue three different ways. All gave the correct slice; only the third had no warning message.

$ perle 'use constant X => (1,3); my @y = "a".."z"; say for @y[X]'
Scalar value @y[X] better written as $y[X] at -e line 1.
b
d
$ perle 'use constant X => (1,3); my @y = "a".."z"; say for @y[+X]'
Scalar value @y[+X] better written as $y[+X] at -e line 1.
b
d
$ perle 'use constant X => (1,3); my @y = "a".."z"; say for @y[X()]'
b
d
[download]

I then looked at these sections of constant: "List constants" and "CAVEATS".

The first of those (see the WEEKDAYS example) suggested a fourth way to try; this gave the correct slice and no error:

$ perle 'use constant X => 1,3; my @y = "a".."z"; say for @y[(X)]'
b
d
[download]

The second has:

"List constants are not inlined unless you are using Perl v5.20 or higher."

This perhaps suggests why [X] (and probably [+X]) had problems; although I'm very much guessing here and don't really know what's going on under the hood.