# Perl {#sec-language-perl}

## Running Perl programs on the shell {#ssec-perl-running}

When executing a Perl script, it is possible you get an error such as `./myscript.pl: bad interpreter: /usr/bin/perl: no such file or directory`. This happens when the script expects Perl to be installed at `/usr/bin/perl`, which is not the case when using Perl from nixpkgs. You can fix the script by changing the first line to:

```perl
#!/usr/bin/env perl
```

to take the Perl installation from the `PATH` environment variable, or invoke Perl directly with:

```ShellSession
$ perl ./myscript.pl
```

When the script is using a Perl library that is not installed globally, you might get an error such as `Can't locate DB_File.pm in @INC (you may need to install the DB_File module)`. In that case, you can use `nix-shell` to start an ad-hoc shell with that library installed, for instance:

```ShellSession
$ nix-shell -p perl perlPackages.DBFile --run ./myscript.pl
```

If you are always using the script in places where `nix-shell` is available, you can embed the `nix-shell` invocation in the shebang like this:

```perl
#!/usr/bin/env nix-shell
#! nix-shell -i perl -p perl perlPackages.DBFile
```

## Packaging Perl programs {#ssec-perl-packaging}

Nixpkgs provides a function `buildPerlPackage`, a generic package builder function for any Perl package that has a standard `Makefile.PL`. It’s implemented in [pkgs/development/perl-modules/generic](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/perl-modules/generic).

Perl packages from CPAN are defined in [pkgs/top-level/perl-packages.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/perl-packages.nix) rather than `pkgs/all-packages.nix`. Most Perl packages are so straight-forward to build that they are defined here directly, rather than having a separate function for each package called from `perl-packages.nix`. However, more complicated packages should be put in a separate file, typically in `pkgs/development/perl-modules`. Here is an example of the former:

```nix
{
  ClassC3 = buildPerlPackage rec {
    pname = "Class-C3";
    version = "0.21";
    src = fetchurl {
      url = "mirror://cpan/authors/id/F/FL/FLORA/Class-C3-${version}.tar.gz";
      hash = "sha256-/5GE5xHT0uYGOQxroqj6LMU7CtKn2s6vMVoSXxL4iK4=";
    };
  };
}
```

Note the use of `mirror://cpan/`, and the `pname` and `version` in the URL definition to ensure that the `pname` attribute is consistent with the source that we’re actually downloading. Perl packages are made available in `all-packages.nix` through the variable `perlPackages`. For instance, if you have a package that needs `ClassC3`, you would typically write

```nix
{
  foo = import ../path/to/foo.nix {
    inherit
      stdenv
      fetchurl # ...
      ;
    inherit (perlPackages) ClassC3;
  };
}
```

in `all-packages.nix`. You can test building a Perl package as follows:

```ShellSession
$ nix-build -A perlPackages.ClassC3
```

To install it with `nix-env` instead: `nix-env -f. -iA perlPackages.ClassC3`.

So what does `buildPerlPackage` do? It does the following:

1. In the configure phase, it calls `perl Makefile.PL` to generate a Makefile. You can set the variable `makeMakerFlags` to pass flags to `Makefile.PL`
2. It adds the contents of the `PERL5LIB` environment variable to `#! .../bin/perl` line of Perl scripts as `-Idir` flags. This ensures that a script can find its dependencies. (This can cause this shebang line to become too long for Darwin to handle; see the note below.)
3. In the fixup phase, it writes the propagated build inputs (`propagatedBuildInputs`) to the file `$out/nix-support/propagated-user-env-packages`. `nix-env` recursively installs all packages listed in this file when you install a package that has it. This ensures that a Perl package can find its dependencies.

`buildPerlPackage` is built on top of `stdenv`, so everything can be customised in the usual way. For instance, the `BerkeleyDB` module has a `preConfigure` hook to generate a configuration file used by `Makefile.PL`: