Again, this problem was just too easy. I wonder why this one was even given actually, because in something like Scheme, this would also be very easy.

Anyways, here it is. Nothing fancy here:

∇R←PEEIGHT;S
S←'73167176531330624919225119674426574742355349194934'
S←S,'96983520312774506326239578318016984801869478851843'
S←S,'85861560789112949495459501737958331952853208805511'
S←S,'12540698747158523863050715693290963295227443043557'
S←S,'66896648950445244523161731856403098711121722383113'
S←S,'62229893423380308135336276614282806444486645238749'
S←S,'30358907296290491560440772390713810515859307960866'
S←S,'70172427121883998797908792274921901699720888093776'
S←S,'65727333001053367881220235421809751254540594752243'
S←S,'52584907711670556013604839586446706324415722155397'
S←S,'53697817977846174064955149290862569321978468622482'
S←S,'83972241375657056057490261407972968652414535100474'
S←S,'82166370484403199890008895243450658541227588666881'
S←S,'16427171479924442928230863465674813919123162824586'
S←S,'17866458359124566529476545682848912883142607690042'
S←S,'24219022671055626321111109370544217506941658960408'
S←S,'07198403850962455444362981230987879927244284909188'
S←S,'84580156166097919133875499200524063689912560717606'
S←S,'05886116467109405077541002256983155200055935729725'
S←S,'71636269561882670428252483600823257530420752963450'

⍝ Find the greatest product of five consecutive digits in S
R←⌈/5×/⎕FI ((2×⍴S)⍴1 0)S
∇