I'd look up some other modules on CPAN and use those as a starting point. E.g. the first lines of Makefile.PL from the most recent Test::Differences module looks like this:

use ExtUtils::MakeMaker;

WriteMakefile(
    NAME          => "Test::Differences",
    VERSION_FROM  => "Differences.pm",
    PREREQ_PM     => {
        "Text::Diff" => 0.34,
    },
);
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Which basically says that Test::Differences requires Text::Diff in version 0.34 (or higher).

See the ExtUtils::MakeMaker documentation for more information.