As MPelletier notes, there is no statement operator but you can use the "assignment trick": assign the result of a statement to a variable when you use it, the first time (from right to left) that you use it. Eg:
Instead of this:
a =: 15
f =: (+/) % # i.a
g =: (-/) % # i.a
h =: ((-:g) * (+:f)) ^ ((-:f) * (+:g))
you can write this:
h =: ((-:g) * (+:f)) ^ ((-: f =. (+/) % #i.a) * (+: g =. (-/) % # i.a=.15))
Or, instead of:
mean =: (+/) % #
f =: mean i.15
this:
f =: (mean =: (+/) % #) i.15