use 5.014;
use strict;
use Devel::Peek;

my $x = 'not a number';
say 0 + $x;
Dump $x;
__END__
0
SV = PVNV(0x1b87050) at 0x1ba83b8
  REFCNT = 1
  FLAGS = (PADMY,POK,IsCOW,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x1bafe30 "not a number"\0
  CUR = 12
  LEN = 14
  COW_REFCNT = 1
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By your definition, smart-matching against $x would do a numeric comparision, because it has a numeric slot. There's your WTF right away.