Juxtaposition in ML is function application: f x y = f(x,y)
Although you're free to discount ML and its ilk (OCaml, Haskell) as non-mainstream.
This isn't quite the same juxtaposition as math, though. In math you just put single letters next to each other and it's multiplication. But in any programming language enforcing single-letter names would be crazy....
Juxtaposition Mathematica works pretty much the same way as in Fortress. When you multiply a number with a variable you don't even need the whitespace (the pretty printer will however make a "half-space" to ease reading).
String interpolation in shell, Python etc; and format specification in C-style printf.