Your subroutine is acting as programmed. You get a single array of arguments to work with in your subroutine.

&writefile("file.txt", @array);
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arguments as writefile sees them:
file.txt, 1rst array element, second array element, third array element, etc...
So you are getting the file name as the first argument($_[0]), first array element as the second argument($_[1]), second array element as the third argument($_[2])

What you could do to be clearer and in similar style:

sub writefile {
  my ($filename, @array) = @_;

  open(FILE, ">$filename") || return ($!);
  print FILE @array; #nothing in between?
  close(FILE);
  return 0;
}
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Though I would pass a reference to the array since that would be quicker. Hint: \@array and @$arref