You are close. All arguments to a subroutine are passed as a single ordered array. That array is accessed in the subroutine by @_.
 my ($average_num, @nums) = @_; means take the first element of @_ and make a copy of it into the subroutine's variable $average_num, then copy all of the elements that are "left over" to the subroutine's array of @nums.

my ($average_num, $first_num, @rest_of_nums) = @_; is possible. But you can't have my ($average_num, @nums, $last_num) = @_; because @nums will consume all entries that are left in @_ and $last_num will be undefined.

The @_ array is very special and some tricks can be played in certain circumstances. None of which apply in 99.9% of Perl code.

Consider the following:

use strict;
use warnings;

my @numbers = qw(4 12 18 21 35);

my $average = find_average(\@numbers);
print "The average found for this list is: $average \n" ;

## find_average will cause a fatal error if
## there are no numbers in the array.

sub find_average  
{
   # subroutine gets a sinle value which is a reference
   # to an array, not an arry itself. This is much, much
   # faster if say the array has say 1,000 elements.
   
   my $num_array_ref = shift;
   
   # could be: my ($num_array_ref) = @_;
   # The shift operation is very slightly faster if
   # only one value is involved.
   
   my $sum;   # no need to set $sum=0; 
              # but I wouldn't quibble if you did that.
   
   foreach my $num (@$num_array_ref) 
   {
      $sum += $num;
   }
   
   my $avg = $sum / @$num_array_ref;
   return $avg;
}

__END__
The average found for this list is: 18
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Update: I know that some Monks would say that the "return $avg" statement is extraneous because by default, Perl will return the value of the last statement in the subroutine. I don't want to relitigate that except to say that an explicit return statement is in my opinion good practice.