s/(\w+)\s+tacos/$1 yummy tacos/g;

##</code><code>##

s/(\w+)\s+(<\w\w>)?tacos/$1 yummy $2tacos/g;

##</code><code>##

$string = [
  ['I','<5b>'],
  [' '],
  ['l','<5c>'],
  ['i'],
  ['k'],
  ['e'],
  [' '],
  ['t','<5d>'],
  ['a'],
  ['c'],
  ['o'],
  ['s']
]