How to get refund from accidental app subscription purchase

You must first cancel any subscription automatic renewal (if you simply delete an app this does not cancel subscriptions):

View, change, or cancel your subscriptions [subscription automatic renewal] - "Your membership or subscription might renew if you don't cancel your membership or subscription more than 24 hours before your renewal date." At this stage the renewal is considered a purchase. For a new subscription the sale is probably effective from when you first agree to subscribe. Canceling a subscription does not automatically start a refund since the existing subscription will normally continue until the end of its term and then just not renew. - https://support.apple.com/HT202039

If you wish a refund: Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no.

Request a refund for an App Store or iTunes Store purchase - does not apply to "recurring charges" which I take to mean subscriptions, so you will have to make a special request using one of the contact links below.

https://support.apple.com/HT204084

Contact iTunes Support:

https://www.apple.com/emea/support/itunes/contact.html

https://support.apple.com/country-selector/itunes