use strict;
use warnings;
use 5.012;

use Safe;

our $x = 10;

if($ARGV[0]) {
    my $code = $ARGV[0];

    my $compartment = Safe->new;
    $compartment->share('$x');
    $compartment->permit_only(qw{ print readline});
    $compartment->reval($code);

    if ($@) {
        print "Unsafe code detected: $@";
    }

}

--output:--
$ perl 2.pl 'print "$x\n";'
Unsafe code detected: 'private value' trapped by operation mask at (eval 5) line 1.