Function arguments in Nix

{ stdenv, fetchurl, perl }:

This line says that this expression is a function taking three arguments, stdenv, fetchurl, and perl.

We use these arguments in Hello, Nix! to indicate that the Nix expression there needs stdenv (a standard environment including common file manipulation tools and a compiler), fetchurl (a function which downloads files), and perl (an interpreter for the Perl programming language) in scope in order to build the package.