The first line declares an alias $var for the global variable $var in the current package. The second line makes it an alias of the $var variable in the main package.

Therefore, it only makes sense to use this in a package different to main. For example:

#!/usr/bin/perl
use warnings;
use strict;

our $var;
$var = 42;

{   package My;

    our $var;
    *var = \$main::var;
    print $var, "\n";
    $var = 84;
}

print $var, "\n";
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