Instead of relying on egrep, sed and multiline shell commands being aailable, why don't you just run that script once (or after each checking) and output a fresh Makefile.PL?

A Makefile.PL is not hard to write and it immediately enables many tools to automatically test and install your distribution and all of its (listed) prerequisites.

ExtUtils::Makemaker lists a very simple Makefile.PL:

use ExtUtils::MakeMaker;

  WriteMakefile(
      NAME              => "Foo::Bar",
      VERSION_FROM      => "lib/Foo/Bar.pm",
  );
[download]

You would likely expand that with the appropriate PREREQ_PM section:

use ExtUtils::MakeMaker;

  WriteMakefile(
      NAME              => "Foo::Bar",
      VERSION_FROM      => "lib/Foo/Bar.pm",
      PREREQ_PM => {
        # Require other prerequisites
        "Acme::Buffy" => 0,
        ...
    }
  );
[download]

If you're feeling extra crafty, you could generate that Makefile.PL automatically, but as such mechanisms usually fail or require special cases, I prefer manually maintaining such lists.