"no autovivication" pragma fails with grep in Perl

It's not grep but the aliasing of the values in your hash slice that causes this.

This has the same effect:

for (@h{@fake_keys}){
 my $t = $_;
}

You can work around it by copying the slice:

use strict;
use warnings FATAL => 'all';
use feature 'say';
use autodie ':all';
use DDP;

my %h = (
    'a' => 1,
    'b' => 2,
);
p %h; # pretty print the original hash
my @fake_keys = ('x', 'y', 'z');
if (grep {defined} @{[@h{@fake_keys}]}) {
    say "found fake keys";
}
p %h; # show that 'x', 'y', & 'z' are added as undef keys

Or access each value in the grep block like shown by Ikegami.

Solution:

if (grep { defined($h{$_}) } @fake_keys) {
   say "found fake keys";
}

Explanation follows.

---

Autovivification, as used in the Perl documentation, is the creation of anon vars and references to them when an undefined scalar is dereferenced.

For example, autovivification dictates that $x->[0] is equivalent to ( $x //= [] )->[0].

For example, autovivification dictates that $h{p}{q} is equivalent to ( $h{p} //= {} )->{q}.

There is no dereferencing in your code, so autovivifaction couldn't possibly occur in your code, so no autovivification; doesn't help.

---

What you have in your code is a hash element used as an lvalue. "lvalue" means assignable value. It's named after the fact that such expressions are usually found on the left of assignments.

  $h{key} = ...;
# ^^^^^^^
# lvalue

But they are also found elsewhere in Perl.

for ($h{key}) {
   # ^^^^^^^
   # lvalue
}

map { } $h{key}
      # ^^^^^^^
      # lvalue

grep { } $h{key}
       # ^^^^^^^
       # lvalue

some_sub($h{key});
       # ^^^^^^^
       # lvalue

This is because the block of each of looping constructs can modify the items being processed by modifying $_, and subs can modify their arguments my modifying the elements of @_.

for ($h{key}) {
   $_ = uc($_);                 # Modifies $h{key}
}

grep { $_ = uc($_) } $h{key}    # Modifies $h{key}  # Bad practice, but possible.

map { $_ = uc($_) } $h{key}     # Modifies $h{key}

sub some_sub {
   $_[0] = uc($_[0]);           # Modifies $h{key}
}

some_sub($h{$k});

For that to be possible, $h{$k} must exist before the loop body is entered or the sub is called.

$ perl -M5.010 -e'for ($h{key}) { }  say 0+keys(%h);'
1

Sub calls use expensive magic to avoid this.

$ perl -M5.010 -e'sub f { }  f($h{key});  say 0+keys(%h);'
0

But grep and map do not.

$ perl -M5.010 -e'grep { 1 } $h{key};  say 0+keys(%h);'
1

$ perl -M5.010 -e'map {; } $h{key};  say 0+keys(%h);'
1