The best way to stop registration spammers on register kind?

I’ve a perl primarily based customized web CMS script and it has registration kind with google recaptcha. However it appears even google capctha doesnt work towards registration spammers.
These registration spammers registers with automated , pretend e-mail addresses with pretend usernames 10 person a day. The programmer who wrote this script doesnt assist.

What are the most effective methods to stop from them?

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