In which year were the first perforated stamps issued?

February 1854. The first perforated stamp was the British penny red.

Source: New Shell Boook of Firsts.

The Irishmen Henry Archer, who worked for the railways, is considered to be the inventor of perforation. He sold his patents to the British Ministry of Finance. Archer completed thereafter his invention. Some prototypes were build. The greatest problem was how to get rid of the remains from the holes. These little pieces of paper caused the machine to a standstill over and over.

Stamps which were part of these trials were sold at the post offices during the years 1848-1850. These are regarded to be the first perforated stamps.

In 1850 Archer finished his trials. The State Printing works further developed the perforation machines made by David Napier & Sons. From 1854 onwards only perforated stamps were issued in Great Britain. https://www.stanleygibbons.com/blog/early-perforation-trials-and-henry-archer-perforated-stamps/