Nice :). Your post inspired me to try something with Lvalue subroutines, I think this works quite well:

use v5.20;
use Operator qw{in};
use Variables qw{X Y Z};

my @list = 1..10;

(X, Y) = (1, 11);
say X." in @list" if X in @list; 
say Y." not in @list" unless Y in \@list;

Z = 'A';
say Z." in list" if Z in ['A'..'C'];
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package Variables;
use v5.20;

our %V;

sub import
{
  shift;
  my $package = caller;
  no strict "refs";
  for my $var (@_)
  {
    *{"$package::$var"} = sub:lvalue { @_ ? $_[0]->($V{$var}) : $V{$va
+r} };
  }
}
1;
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package Operator;
use v5.20;
use List::Util qw{any none};
use Exporter 'import';
our @EXPORT_OK = 'in';


sub in(+)
{
  my $list = shift;
  return sub { any { $_ eq $_[0] } @$list };
}
1;
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1 in 1 2 3 4 5 6 7 8 9 10
12 not in 1 2 3 4 5 6 7 8 9 10
A in list
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Edit: fixed the broken link for lvalue subroutines thanks to shmem.

Edit: $V{$var} is what I meant, not $V::{$var}