It's doing a couple things, although only one of them is relevant to the execution of code:

• As mentioned, it's returning 1 if there are any items in @value, and false otherwise.
• It's wreaking havoc on Vim's syntax highlighting.
• It's wreaking havoc on my own poor brain trying to parse "not not (scalar) @value".

I'd probably use a more explicit test, as the following example shows (lexical variables added for my own peace of mind):

use Modern::Perl;

sub mean_test {
    my @value = @_;

    return !!@value;
}

sub nicer_test {
    my @values = @_;

    return scalar @values > 0;
}

say "Mean: Empty: ", mean_test();
say "Mean: One Item: ", mean_test("sweet!");
say "Mean: Two Items: ", mean_test("dude!", "sweet!");

say "Nicer: Empty: ", nicer_test();
say "Nicer: One Item: ", nicer_test("sweet!");
say "Nicer: Two Items: ", nicer_test("dude!", "sweet!");
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It evaluates the same, as far as I can tell.

Mean: Empty:
Mean: One Item: 1
Mean: Two Items: 1
Nicer: Empty:
Nicer: One Item: 1
Nicer: Two Items: 1