Variable and package names are case sensitive. Simply replace that with $Foo::x and $Bar::x

use strict and use warnings would've alerted you to the problem.

package Foo;
our $x = 1;

package Bar;
our $x = 2;

package main;

use strict;
use warnings;

our $x = 3;
print "x = $x\n";
print "Foo::x = $foo::x\n";
print "Bar::x = $Bar::x\n";

$x = 4;
print "x = $x\n";
print "Foo::x = $Foo::x\n";
print "Bar::x = $Bar::x\n"
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