That's what it is. Regex matching makes a copy of the scalar for use by $&, $1, etc. It uses the COW mechanism if possible. It doesn't for OOK scalars.

After one pass of test1's loop:

SV = PVMG(0x5bb22b8c3b80) at 0x5bb22b9beae8
  REFCNT = 1
  FLAGS = (SMG,POK,IsCOW,pPOK)    <-- IsCOW: String buffer is shared
  IV = 0
  NV = 0
  PV = 0x5bb22b9ea230 "..."
  CUR = 99999
  LEN = 100001
  COW_REFCNT = 1
  MAGIC = 0x5bb22b9c48f0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_FLAGS = 0x40
      BYTES
    MG_LEN = -1
[download]

After one pass of test2's loop:

SV = PVMG(0x5bb22b8c3bb0) at 0x5bb22b9cc1f0
  REFCNT = 1
  FLAGS = (SMG,POK,OOK,pPOK)    <-- !IsCOW: String buffer isn't shared
  IV = 0
  NV = 0
  OFFSET = 1
  PV = 0x5bb22ba028e1 ( "\x01" . ) "..."
  CUR = 99999
  LEN = 100001
  MAGIC = 0x5bb22b9c48f0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = PERL_MAGIC_regex_global(g)
    MG_FLAGS = 0x40
      BYTES
    MG_LEN = -1
[download]

That means a copy of the string buffer must have been made. By the end of the loop, a copy has been done 99,999 times (once per successful regex match).

You can run into the same issue with foreign string buffers (e.g. a memory-mapped file). See why is Perl File::Map so slow compared to File::Slurp?