IV - holds signed int, unsigned int or ref
NV - holds float
PV - holds string
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And then there are fields to support magic lvalues and general magic.

Not every field is available at all times. SVs (scalars) have a head which contains a pointer to the body. The body is automatically upgraded when needed (e.g. my $x = 1; $x += 0.1;, tie my $x, ..., etc).

And even if the field is available, not all values are available at all times. Flags indicate which fields are initialised and what they contain.

$ perl -MDevel::Peek -e'
   my $x;    Dump $x;
   $x = 123; Dump $x;
  "$x";      Dump $x;
  ++$x;      Dump $x;
  $x=undef;  Dump $x;
'

Fresh var, no body, no content:

SV = NULL(0x0) at 0x88c6920
  REFCNT = 1
  FLAGS = (PADMY)

After assigning int,
body upgraded to IV (has field IV), and
IV field contains a number (IOK):

SV = IV(0x88c691c) at 0x88c6920
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 123

After using content as string,
body upgraded to PVIV (has fields IV and PV),
IV field contains a number (IOK), and
PV field contains a string (POK):

SV = PVIV(0x88c56ac) at 0x88c6920
  REFCNT = 1
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 123
  PV = 0x88e7710 "123"\0
  CUR = 3
  LEN = 4

After using content as a number,
IV field contains a number (IOK), and
PV field isn't being used (!POK):

SV = PVIV(0x88c56ac) at 0x88c6920
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 124
  PV = 0x88e7710 "123"\0
  CUR = 3
  LEN = 4

After clearing the contents,
IV field isn't being used (!IOK), and
PV field isn't being used (!POK):

SV = PVIV(0x88c56ac) at 0x88c6920
  REFCNT = 1
  FLAGS = (PADMY)
  IV = 124
  PV = 0x88e7710 "123"\0
  CUR = 3
  LEN = 4
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