I can't call "require".

You seem to have lost track of what you said because you're the one who used require. You said

use if eval { require mod; 1 }, qw' constant haveMod 1 ';
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is ugly because constant is being used. I pointed out that it's simply not true. Not only does that code not work, but it can be written as

use constant haveMod => eval { require mod; };
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I am sniffing to see what PMs were loaded before the "use" that loads my PM.

Asked and answered. Or if you want to check for multiple modules,

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