Devel::Peek shows you the internal details of scalars.

>perl -e"use Devel::Peek; $x='123'; Dump($x); 0+$x; Dump($x);"
SV = PV(0x2369cc) at 0x182a22c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x23fee4 "123"\0
  CUR = 3
  LEN = 4
SV = PVIV(0x182005c) at 0x182a22c
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 123
  PV = 0x23fee4 "123"\0
  CUR = 3
  LEN = 4
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illguts explains this area.

And then there's -MO=Concise to show you the opcode tree.

>perl -MO=Concise -e"for (1..2) { print qq{Hello World\n} }"
g  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 2 -e:1) v ->3
f     <2> leaveloop vK/2 ->g
7        <{> enteriter(next->c last->f redo->8) lKS/8 ->d
-           <0> ex-pushmark s ->3
-           <1> ex-list lK ->6
3              <0> pushmark s ->4
4              <$> const[IV 1] s ->5
5              <$> const[IV 2] s ->6
6           <#> gv[*_] s ->7
-        <1> null vK/1 ->f
e           <|> and(other->8) vK/1 ->f
d              <0> iter s ->e
-              <@> lineseq vK ->-
8                 <;> nextstate(main 1 -e:1) v ->9
b                 <@> print vK ->c
9                    <0> pushmark s ->a
a                    <$> const[PV "Hello World\n"] s ->b
c                 <0> unstack v ->d
-e syntax OK

>perl -MO=Concise,-exec -e"for (1..2) { print qq{Hello World\n} }"
1  <0> enter
2  <;> nextstate(main 2 -e:1) v
3  <0> pushmark s
4  <$> const[IV 1] s
5  <$> const[IV 2] s
6  <#> gv[*_] s
7  <{> enteriter(next->c last->f redo->8) lKS/8
d  <0> iter s
e  <|> and(other->8) vK/1
8      <;> nextstate(main 1 -e:1) v
9      <0> pushmark s
a      <$> const[PV "Hello World\n"] s
b      <@> print vK
c      <0> unstack v
           goto d
f  <2> leaveloop vK/2
g  <@> leave[1 ref] vKP/REFC
-e syntax OK
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You can locate the code for each op by searching for pp_op_name. It'll be in one of the pp*.c files. For example, the code for enteriter is in pp_enteriter in pp_ctl.c.

This provides a good entry into perl since you're looking at code whose function you already know.