Similar to parv's comment: A function argument list is a list (and assigned to the magical @_ array), and Perl flattens lists.

Win8 Strawberry 5.8.9.5 (32)  Wed 06/09/2021 17:59:05
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
use Data::Dump qw(dd);

sub fun { dd \@_; }

fun('foo');
fun('foo',,,,,,,,);
# fun(,,,,,,,,'foo');  # syntax error!
# fun(,'foo');  # syntax error!
fun('foo','bar');
fun('foo',,,,,,,'bar');
fun('foo' => => => => => => 'bar');
^Z
["foo"]
["foo"]
["foo", "bar"]
["foo", "bar"]
["foo", "bar"]
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Update: Note that fun( => 'foo') is also a syntax error.