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.

> The second has: "List constants are not inlined unless you are using Perl v5.20 or higher."

ah thanks, that's the relevant part I missed. :)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

I get a warning for FL2 Scalar value @row[FL2] better written as $row[FL2] (ignoring that its a constant _list_)

$ ./Porting/bisect.pl --target=miniperl --start=v5.16.3 --end=v5.26.0 
+--expect-fail -e 'use warnings FATAL=>"all"; sub X () {2,3}; my @a=("
+a".."z"); print @a[X]'
...
commit 429a25554a608b64c0ee46c1ffe19bab3718a3c8
Author: Father Chrysostomos <sprout@cpan.org>
Date:   Sat Sep 14 17:23:11 2013 -0700

    Reduce false positives for @hsh{$s} and @ary[$s] warnings
    
    This resolves tickets #28380 and #114024.
    
    Commit 95a31aad5 did something similar to this for the new %hash{.
+..}
    syntax.  This commit extends it to @ slices and combines the two
    code paths.
    
    The heuristics in toke.c can easily produce false positives.  So t
+he
    op is flagged as being a candidate for the warning.  Then when op.
+c
    has the op tree available, it examines it to see whether the heuri
+stic
    may have been a false positive.
    
    This avoids bugs with qw "foo bar baz" and sub calls triggering
    the warning.
    
    The source code is no longer available for the warning, so we reco
+n-
    struct it from the op tree, skipping the subscript if it is anythi
+ng
    other than a const op.
    
    This means that @hash{$foo} comes out as @hash{...} and @hash{foo}
+ as
    @hash{"foo"}.  It also meeans that @hash{"]"} is displayed correct
+ly
    instead of as @hash{"].
    
    Commit 95a31aad5 also modified the heuristic for %hash{...} to exe
+mpt
    qw altogether.  But it did not exempt it if it was preceded by a t
+ab.
    So this commit rectifies that.
    
    This commit also improves the false positive detection by exemptin
+g
    any ops returning lists that can get past toke.c’s heuristic.  I w
+ent
    through the entire list of ops, but I may have missed some.
    
    Also, @ slices on the lhs of = are exempt, as they change the cont
+ext
    and are hence actually useful.

$ git tag --contains 429a25554a608b6
... v5.20.0 ...
[download]

Deparsing the code shows that FL and FL2 are called as functions @a[FL()] = @row[FL2()];

$ ./Porting/bisect.pl --start=v5.16.3 --end=v5.26.0 -e '`$^X -Ilib -MO
+=Deparse -e "use constant X=>3,4; \@x[X]"`=~/\@x\[3, 4\]/ and die'
...
commit f815dc14d7c5540dfb5d02d001e0101c6266f281
Author: Father Chrysostomos <sprout@cpan.org>
Date:   Sun Jun 30 00:20:33 2013 -0700

    Inline list constants
    
    These are inlined the same way as 1..5.  We have two ops:
    
      rv2av
        |
        `-- const
    
    The const op returns an AV, which is stored in the op tree, and th
+en
    rv2av flattens it.

$ git tag --contains f815dc14d7c5540d
... v5.20.0 ...

# *Update:* making sure it's not just the Deparse output that changed.
+..
$ ./Porting/bisect.pl --start=v5.19.0 --end=v5.21.0 -e '`$^X -Ilib -MO
+=Concise -e "use constant X=>3,4; \@x[X]"`=~/entersub/ or die'
...
commit f815dc14d7c5540dfb5d02d001e0101c6266f281
[download]

Thanks, bookmarked to learn more about bisect! :)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

Thanks, bookmarked to learn more about bisect! :)

Once you get into it, it's not too difficult... see perldoc Porting/bisect-runner.pl.

Update: Better link.

Update 2: How to Bisect Perl

I'm not sure what you expect. Both output lines are the same:

$ perl 1218891.pl 
Scalar value @row[FL2] better written as $row[FL2] at 1218891.pl line 
+17.
      e                         n w                
      e                         n w                
$ perl -v | head

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-li
+nux-thread-multi
(with 25 registered patches, see perl -V for more detail)

Copyright 1987-2012, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
[download]

> I'm not sure what you expect.

of course it's the same output.

The performance depends on the generated opcodes.

I wasn't aware that constant folding of lists wasn't implemented in "oldish" Perl versions.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

The performance depends on the generated opcodes.

Thanks - this was the bit I missed from the OP. It is now clear to me why this was of a concern to you.

Testing with 5.24 shows constant folding of lists to work:

X:\>perl -MO=Deparse -e"use constant L=>1,2,3; print @a[L]"
use constant ('L', 1, 2, 3);
print @a[1..3];
-e syntax OK
[download]

seems like constant folding of lists was only added "some years" ago. ;-)

update

for comparison 5.18

D:\>perl -MO=Deparse -e"use constant L=>1,2,3; print @a[L]"
use constant ('L', 1, 2, 3);
print @a[L()];
[download]