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!^ᛦ 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^ᛦ programming language) in scope in order to build the package.