I'm not quite sure what you're getting at, but you can certainly get the global value from anywhere, without using local. Observe:

# main scope
$fred = 1; #global
{ # outer scope
    my $fred = 2;
    #...
    { #inner scope
        print $main::fred; # prints the global 1
    }
}
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The best rule of thumb is to use my unless you have some particular reason to use something else.