From 'perldoc -f our'

An "our" declaration declares a global variable that will be visible a
+cross its entire lexical scope, even across package
boundaries.
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So any variable initialized with my or our is in this case within file scope. Wrap the instantiation of $d within a block to make the scope smaller:

package A;
package B;
{ our $d = 1; }
package C;
package D;
# now you have to use the package name to access the var
print $B::d;
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