This should do what you seem to be trying to do: (untested)

use strict;
use warnings;

sub counter {
  my @nums = (1..500);
  our $add = 0;
  for my $num (@nums) { $add += $num }
}

{
  our $add;
  say $add;
}
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This uses our to establish lexical aliases to a global variable "add" briefly as needed. For a small program, or within a module, this can be a useful technique, but please be careful not make gratuitous uses of global variables across larger programs.

If you need more than one counter, objects are a better approach: (also untested)

package Acme::Sample::Counter;

sub new {
  my $class = shift;
  my $ob = (shift || 0);
  bless \$ob, $class;
}

sub count {
  my $self = shift;
  $$self += (shift || 1);
}

sub read {
  my $self = shift;
  return $$self;
}
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Used like so:

my $counter = new Acme::Sample::Counter ();
$counter->count;      # add 1
$counter->count(10);  # add 10
say $counter->read;
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Note however, that $counter->count(0) adds 1! Think about why this is so.