Marto can only judge from what you tell us and you didn't tell us what you found out already.

What you could do now would be to find out if the error message is generated by different ip numbers or always the same. You can do that with a print $stuff[0]; directly before line 23. The lastz ip you see printed must be the offending one

If it is always the same number, make a whois query yourself to check what is different with this number. If not, you may have run up against Denial-of-Service prevention measures of the service providers. Maybe they don't like bots to check hundreds of addresses per second