Upon further reflection, $Config{nv_overflows_integers_at} doesn't tell me anything I didn't know. It still doesn't give me something I can use to determine if an argument that looks like an integer will fit in an integer. I will give it credit for obscurity, though:-)

  DB<1> use Config

  DB<2> say $Config{nv_overflows_integers_at}
256.0*256.0*256.0*256.0*256.0*256.0*2.0*2.0*2.0*2.0*2.0
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If I eval it, I get something that is, pretty much by definition, something that won't fit in an integer. But, as we have seen, if I compare it using > to ~0, a valid integer, it may not compare high.