our $var; # Declare a variable in the current package.
*var = \$main::var; # Create an alias in the current package to packag
+e main's $var;
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Observe the following:

package Foo;
our $var = 42;
print __PACKAGE__, ": $var\n";
print \$var, "\n";

package main;
*var = \$Foo::var;
print __PACKAGE__, ": $var\n";
print \$var, "\n";
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The output:

Foo: 42
SCALAR(0x559ec7e7b6a0)
main: 42
SCALAR(0x559ec7e7b6a0)
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This is aliasing; you're creating in main a symbol that refers to the same variable that is known in Foo as $var. Notice they share the same memory address. (It will be a different address on your system).

In your example code you were creating $var in the current package and aliasing it to $main::var (the $var that belongs to main::). The example I gave is going in the opposite direction because it's easier to demonstrate. But the concept is the same; you're setting up a symbol that refers to the same thing that is known in another package namespace simultaneously.

This is what Exporter does.