Some snakes can be a bit touchy after they shed. In general, though, it is best not to even touch the snake's head at all... When confronted by a predator, they usually go for the snake's head first so it often frightens snakes to be touched around their heads (even by familiar people). It's actually a lot more unusual for a snake not to jerk its head back when touched than to behave in the manner you're describing... Sounds like you have a pretty tame boa. Hope this helps.
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- Natalie
(San Francisco Bay Area)