use ExtUtils::MakeMaker;

my %prereqs = ( 
  "Blah::Blah" => 0,
  "Foo::Bar"   => 1.0,
);

eval{require Some::Module};
if ($@) {
    my $reply = prompt("Some::Module not installed. Install (Y/n)", "Y
+");
    $prereqs{"Some::Module"} = 0 unless $reply =~ m/n/i;
}

WriteMakefile(
      PREREQ_PM => \%prereqs,
);