callPackage

callPackage is a convenience function in nixpkgs.

Supposing we put the Nix expression in Hello, Nix!ᛦ in a file at ./hello/default.nix,

hello = callPackage ./hello { };

is equivalent to

hello = import ./hello {
  inherit stdenv fetchurl perl;
}

That is, callPackage inherits whatever values are in scope for the arguments of the derivation. Values can be overridden in the curly braces. If we had our own special perl called myPerl, we could say

hello = callPackage ./hello {
  perl = myPerl;
};

or, equivalently,

hello = import ./hello {
  inherit stdenv fetchurl;
  perl = myPerl;
}
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