Had the declaration been "my $n = 0", then for every subroutine call it would start with 0...

Depending on where "my $n = 0" was placed and how.
Using "my $n = 0" outside the subrourtine, placed in a block like so:

 for (1..5){marine()};

{
 my $n=0; # private, persistent variable $n

  sub marine {
    $n += 1;
    print "Hello, sailor number $n!\n";
  }
}
[download]

use 5.010;

for (1..5){marine()}; 

sub marine {
    state $n = 0; # private, persistent variable $n
    $n += 1;
    print "Hello, sailor number $n!\n";
  }
[download]