As an alternative to the state keyword for turning your subroutine into a coroutine (a function that maintains state between calls), here is how you use a closure to effectively maintain a state variable declared outside of the counter subroutine and provides access to the current value of $add via subroutine "getter". In this way, $add is read only and only affected via counter. The ADD_COUNTER_CLOSURE: is simply a label handy in this case for self documenting the existence of the closure itself.

ADD_COUNTER_CLOSURE:
{
  # $add is not accessible outside of closure, but
  # you do the 'getter', "add" which has access to $add
  my $add = 0;
  sub counter {
    my @nums = (1..500); 
    for my $num(@nums) { $add += $num }
  }
  
  # getter
  sub add {
    return $add;
  }
}

# $add is not accessible directly here, must use add subroutine

my $add = add;
print qq{$add\n};

# increment $add via closure()
counter();
$add = add;
print qq{$add\n};
[download]

Output:

0
125250
[download]

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