Nope, it happens because of the aliasing of the hash value inside grep, but I'm not sure how.

#!/usr/bin/perl 

use strict;
use warnings;
use Data::Dumper;

my $h = {
          'a' => 1,
          'b' => 2,
          'c' => 3,
        };

warn Dumper [ $$h{a}, $$h{x}, $$h{b}, $$h{c} ];
warn Dumper [ keys %$h ];

warn Dumper [ grep {$_} $$h{a}, $$h{x}, $$h{b}, $$h{c} ];
warn Dumper [ keys %$h ];

warn !$$h{y};
warn Dumper [ keys %$h ];
[download]

C:/Perl_524/bin\perl.exe  d:/tmp/pm/grep_autovivify.pl 
$VAR1 = [
          1,
          undef,
          2,
          3
        ];
$VAR1 = [
          'b',
          'a',
          'c'
        ];
$VAR1 = [
          1,
          2,
          3
        ];
$VAR1 = [
          'a',
          'x',
          'c',
          'b'
        ];
1 at d:/tmp/pm/grep_autovivify.pl line 19.
$VAR1 = [
          'a',
          'x',
          'c',
          'b'
        ];
[download]

Nope, it happens because of the aliasing of the hash value inside grep, but I'm not sure how.

Interesting effect. I see your output with plain perl v5.22.2 (from 2016) on Linux x86-64 and with Strawberry v5.14.2 (from 2011) on Windows 64 bit. So this seems to be an old, unsolved problem.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

Maybe I should file a bug on that? I will report bug for Debian...