The prototype for vec is three scalars. See the following:

use strict;
use warnings;

print prototype "CORE::vec";
print "\n";

__OUTPUT__
$$$
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Now try the same thing with a few other popular functions for which you can use either an array or a literal list as the 2nd (and subsequent) argument(s).

use strict;
use warnings;

print prototype "CORE::push", "\n";
print prototype "CORE::sprintf", "\n";
print prototype "CORE::pack", "\n";

__OUTPUT__
\@@
$@
$@
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The difference is immediately apparent.

See perldoc -f prototype for an explanation of how to use the prototype function to gain a better understanding of the prototypes of Perl internal "core" functions.