Since you simply want to conditionally include a lexically scoped module at compile-time it sounds like the if module will do the job. If adapted to your example code it looks to work just fine:

BEGIN { $Y::VERSION = 0.1 }

{
  package X;
  use if ( $Y::VERSION < 1 ), "bytes";

  sub l { return length shift }
}

no bytes;

print X::l( chr 3456 );

__output__
3
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