Prior to perl 5.36 it was not possible to identify a boolean true value after it had been copied

This was done by ensuring that the string buffer (PV) of &PL_sv_yes and &PL_sv_no is shared on assignment.

$ 5.34t/bin/perl -MDevel::Peek -e'Dump(!!1); my $x = !!1; Dump($x);'
SV = PVNV(0x55852a177160) at 0x55852a1753c8
  REFCNT = 2147483644
  FLAGS = (IOK,NOK,POK,READONLY,PROTECT,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x558529a84f64 "1"
  CUR = 1
  LEN = 0
SV = PVNV(0x55852a177220) at 0x55852a178770
  REFCNT = 1
  FLAGS = (IOK,NOK,POK,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x55852a1b6010 "1"\0
  CUR = 1
  LEN = 10

$ 5.36t/bin/perl -MDevel::Peek -e'Dump(!!1); my $x = !!1; Dump($x);'
SV = PVNV(0x55d55f1be160) at 0x55d55f1bc3c8
  REFCNT = 2147483644
  FLAGS = (IOK,NOK,POK,IsCOW,READONLY,PROTECT,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x55d55e89b984 "1" [BOOL PL_Yes]
  CUR = 1
  LEN = 0
SV = PVNV(0x55d55f1be220) at 0x55d55f1bf770
  REFCNT = 1
  FLAGS = (IOK,NOK,POK,IsCOW,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x55d55e89b984 "1" [BOOL PL_Yes]
  CUR = 1
  LEN = 0
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is_bool checks if the provided scalar shares a string buffer (PV) with &PL_sv_yes or &PL_sv_no.

Before you had two "levels" of true:

• &PL_sv_yes
• true or false value

After, three:

• &PL_sv_yes
• is_bool
• true or false value