Here is the sentence including the word. It is said by an old man who is cooking some food and planning to give it to a younger person who is in jail.
ようし わしゃあやったるぜ
I am not sure if it is a shorten form of 「やってやる」which means "Doing something for someone who is below than the speaker"
Is it still used nowadays in Kantou area?
Yes, it's short for やってやる. Please see this answer for the list of similar contractions. I think this contraction is common throughout Japan regardless of generation (but it sounds relatively masculine)
And te-form + やる means not only "to do something for someone" but also "dare to do something", "to do something proactively with an active effort", "to do something and show off (the result / one's power)" etc. See: what does てやる mean when it is not used for giving?
The sentence probably just means "I'll do that" (without "for someone").
