That's most likely a bug in the XS implementation of slide.

I looked into the Pure Perl implementation in List::MoreUtils::PP, and it looked fine, it's literally using map

And a test reveals, that it really works like it should.

use v5.12;
use warnings;
use Data::Dumper;
BEGIN { $ENV{LIST_MOREUTILS_PP} = 1 }; # enforce PP version, comment f
+or XS
use List::MoreUtils qw/slide/;

my @x= split',', q(a,a,a,a,b,c,c,a,a,d,e,e,e,e);

print Dumper slide { $a ne $b ? $b : () } "", @x;
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PP version

$VAR1 = 'a';
$VAR2 = 'b';
$VAR3 = 'c';
$VAR4 = 'a';
$VAR5 = 'd';
$VAR6 = 'e';
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XS version

$VAR1 = 'a';
$VAR2 = undef;
$VAR3 = undef;
$VAR4 = undef;
$VAR5 = 'b';
$VAR6 = 'c';
$VAR7 = undef;
$VAR8 = 'a';
$VAR9 = undef;
$VAR10 = 'd';
$VAR11 = 'e';
$VAR12 = undef;
$VAR13 = undef;
$VAR14 = undef;
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