"Did you?"

I did.

Syntax error without "+":

$ perl -MO=Deparse -e 'my @b = @{ reduce { push @$a, $b if !@$a || $b 
+ne $a->[-1]; $a } [], @a };'
syntax error at -e line 1, near "$b if"
-e had compilation errors.
[download]

Syntax OK with "+":

$ perl -MO=Deparse -e 'my @b = @{ +reduce { push @$a, $b if !@$a || $b
+ ne $a->[-1]; $a } [], @a };'
my(@b) = @{do {
    push @$a, $b if not @$a or $b ne $a->[-1];
    $a
}->reduce([], @a);};
-e syntax OK
[download]

The issue isn't a missing + (which doesn't help parse the code correctly); the issue is that you didn't declare reduce (which has a prototype). Add

use List::Util qw( reduce );
[download]

But your note still shows the former (without), not the latter (with). I guess I'm confused.

What you've linked as "your note" wasn't my note; it was an anonymous posting and we may never know who wrote it.

Following your comment, I was curious. Using the verbatim code posted by AM gave "syntax error". Changing reduce to +reduce gave "syntax OK". I hope that clears up any confusion.

I'm running v5.39.3, if that makes a difference.

— Ken