I think it is mainly Data::Dumper being anal: Perl has a builtin constant called PL_Yes PL_sv_yes which is used internally for the boolean true value. The Dumper output is effectively saying that this isn't just any old constant 1, but the one and only PL_sv_yes. For a data structure with many copies I guess you'd get something of a memory saving compared to having many unique SVs.

You can see the difference with Devel::Peek:

perl -MDevel::Peek -wle 'Dump(1); Dump(!!1)'
SV = IV(0x56346851ef48) at 0x56346851ef58
  REFCNT = 1
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
SV = PVNV(0x5634684f6050) at 0x5634684f43c8
  REFCNT = 2147483644
  FLAGS = (IOK,NOK,POK,READONLY,PROTECT,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x563467d5e282 "1"
  CUR = 1
  LEN = 0
[download]

Builtins like this have an artificial refcount around 2^31 to indicate "should never be freed, not even accidentally".

Perl v5.36 introduced an actual boolean type, but that doesn't make any noticeable difference to the value except that Devel::Peek now annotates the PV as "[BOOL PL_Yes]".

Update: corrected the internal name of the SV; thanks @ikegami.