Demonstration of "will not stay shared":

sub outer {
    my ($x) = @_;
    sub inner {
        say $x;
    }
    inner();
}          # Warns Variable "$x" will not stay shared

outer(4);  # Prints 4
outer(6);  # Prints 4!!!
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Here's what's going on.

sub named { ... }
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is more or less the same as

BEGIN { *named = sub { ... }; }
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As such, it only captures once, when the sub is compiled.

In the above code, when outer goes out of scope, $x would normally be cleared. But since the closure still references it, a new $x is a created instead. From this point on, the $x in inner is different from the $x is outer; it "didn't stay shared".