$ cat test3.pl
#!/usr/bin/perl
use strict;
use warnings;

mysub(1);
mysub(2,3,4);
mysub(5,6,7,8,9,0);

sub mysub {
    my ($first, $second, @third, @fourth)=@_;
    print "First: $first, Second: $second, Third: @third, Fourth: @fourth.\n";
}
$ perl test3.pl
Use of uninitialized value $second in concatenation (.) or string at test3.pl line 11.
First: 1, Second: , Third: , Fourth: .
First: 2, Second: 3, Third: 4, Fourth: .
First: 5, Second: 6, Third: 7 8 9 0, Fourth: .