FWIW and I don't think !!1 used to be dualvar (see Scalar::Util ) prior to 5.36.

It's actually a triplevar, and it has been so at least as far back as 5.6, but probably a lot further back.

I don't have 5.6, but here's 5.8.9: 

$ 5.8.9/bin/perl -MDevel::Peek -e'Dump(!!1)'
SV = PVNV(0x55e2de0d1348) at 0x55e2dd477310
  REFCNT = 2147483647
  FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x55e2de0d2320 "1"\0
  CUR = 1
  LEN = 8

$ 5.8.9/bin/perl -MDevel::Peek -e'Dump(!!0)'
SV = PVNV(0x55aeb71e8320) at 0x55aeb5e05da0
  REFCNT = 2147483647
  FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55aeb71e8300 ""\0
  CUR = 0
  LEN = 8

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In reply to Re^4: Why is Dumper returning "!!1" for true file test? by ikegami
in thread Why is Dumper returning "!!1" for true file test? by nysus

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