use if eval { require mod; 1 }, qw' constant haveMod 1 ';

##</code><code>##

use constant haveMod => eval { require mod; };

##</code><code>##

use constant {
   map {
      no strict 'refs';
      ( my $constant = $_ ) =~ s/:://g;
      "have$constant" => ${"${_}::VERSION"}
   } qw(
      Foo::Mod
      Bar::Mod
   );
};