Found another one...

use List::AllUtils qw( singleton );
my @new6invslice = @array[ singleton 0 .. $#array, @slice_idx ];
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If that's not better, we need a better definition of 'better'. (WARNING: possible infinite recursion encountered!)

My comments in all CAPS.

Most importantly - regarding @newinvslice - Ikegami was right to point out that an undef value is not a sufficiently safe criteria (AKA the semi-predicate problem)

I have a solution using the ref of a scalar var which MUST be unique.

A bit clumsy, because Perl has no === operator, but I am open for more elegant suggestions. :)

#!/usr/bin/perl
use strict;                  # https://perlmonks.org/?node_id=11162546
use warnings;
use Data::Dump 'dd';
use Test::More;

$SIG{__WARN__} = sub { die @_ };

# Is there a better way to get an inverse slice than to do something l
+ike:


my @array = ('aa'..'zz');
my @slice_idx = (6,13,42,66,69);
# Slice
my @slice = @array[@slice_idx];
my $expected = [
    "aa" .. "af",
    "ah" .. "am",
    "ao" .. "bp",
    "br" .. "cn",
    "cp",
    "cq",
    "cs" .. "zz",
   ];

dd {
       'array' => \@array,
        'slice_idx' => \@slice_idx,
        'slice' => \@slice,
        'expected' => $expected,
    };

{
    my $TEST = "not in idx hash";

    my %slice_idx;
    @slice_idx{@slice_idx} = ();        # NO MAP NEEDED
    my @invslice_idx = grep { ! exists $slice_idx{$_} } 0 .. $#array;
    # Inverse slice.
    my @invslice = @array[@invslice_idx];


    is_deeply(\@invslice, $expected, $TEST ) or
      diag dd { 'idx' => \@invslice_idx, 'invslice' => \@invslice };
}

{
    my $TEST = "not delete value";

    my @newinvslice = @array;
    my $delete;
    @newinvslice[@slice_idx] = (\$delete) x @slice_idx; # UNIQUE REF V
+ALUE
    @newinvslice = grep { ref $_ ne "SCALAR" or $_ ne  \$delete }  @ne
+winvslice;


    is_deeply(\@newinvslice ,$expected, $TEST)  or
      diag dd { 'newinvslice' => \@newinvslice };
}

{
    my $TEST = "splice backwards";

    my @new3invslice = @array;
    splice @new3invslice, $_, 1 for sort { $a < $b } @slice_idx; # ENS
+URE IDX SORTED

    is_deeply(\@new3invslice, $expected, $TEST)  or
      diag dd  { 'new3invslice' => \@new3invslice };
}


done_testing;
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{
  array     => ["aa" .. "zz"],
  expected  => [
                 "aa" .. "af",
                 "ah" .. "am",
                 "ao" .. "bp",
                 "br" .. "cn",
                 "cp",
                 "cq",
                 "cs" .. "zz",
               ],
  slice     => ["ag", "an", "bq", "co", "cr"],
  slice_idx => [6, 13, 42, 66, 69],
}
ok 1 - not in idx hash
ok 2 - not delete value
ok 3 - splice backwards
1..3
[download]

Updates

Bugfix s/and/or/

How to fix the semi-predicate problem using undef :)

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11162546
use warnings;
$SIG{__WARN__} = sub { die @_ };

# Is there a better way to get an inverse slice than to do something l
+ike:

my @array = ('aa'..'zz');
my @slice_idx = (6,13,42,66,69);
# Slice
my @slice = @array[@slice_idx];

use Data::Dump 'dd'; dd { 'array' => \@array,
  'slice_idx' => \@slice_idx, 'slice' => \@slice };

my %slice_idx = map { $_ => 1 } @slice_idx;
my @invslice_idx = grep { ! exists $slice_idx{$_} } 0 .. $#array;
# Inverse slice.
my @invslice = @array[@invslice_idx];
use Data::Dump 'dd'; dd { 'idx' => \@invslice_idx, 'invslice' => \@inv
+slice };

my @wanted = 0 .. $#array;
@wanted[@slice_idx] = ();
my @new6invslice = @array[ grep defined, @wanted ];
use Data::Dump 'dd'; dd  { 'new6invslice' => \@new6invslice };
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Outputs:

{
  array => ["aa" .. "zz"],
  slice => ["ag", "an", "bq", "co", "cr"],
  slice_idx => [6, 13, 42, 66, 69],
}
{
  idx => [0 .. 5, 7 .. 12, 14 .. 41, 43 .. 65, 67, 68, 70 .. 675],
  invslice => [
    "aa" .. "af",
    "ah" .. "am",
    "ao" .. "bp",
    "br" .. "cn",
    "cp",
    "cq",
    "cs" .. "zz",
  ],
}
{
  new6invslice => [
    "aa" .. "af",
    "ah" .. "am",
    "ao" .. "bp",
    "br" .. "cn",
    "cp",
    "cq",
    "cs" .. "zz",
  ],
}
[download]