I couldn't leave it at that. It seems that Devel::Size actually modifies the array!

>perl -MDevel::Size=total_size -MDevel::Peek -e"my @a; total_size(\@a)
+ if $ARGV[0]; Dump \@a,1" 0
SV = RV(0x239ba8) at 0x239b9c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x1829954
  SV = PVAV(0x23aa60) at 0x1829954
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)

>perl -MDevel::Size=total_size -MDevel::Peek -e"my @a; total_size(\@a)
+ if $ARGV[0]; Dump \@a,1" 1
SV = RV(0x239ba8) at 0x239b9c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x1829954
  SV = PVAV(0x23aa60) at 0x1829954
    REFCNT = 2
    FLAGS = (PADMY,RMG)
    MAGIC = 0x1824014
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

It's similar but different than using $#a:

>perl -MDevel::Size=total_size -MDevel::Peek -e"my @a; $#a if $ARGV[0]
+; Dump \@a,1" 0
SV = RV(0x239260) at 0x239254
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182991c
  SV = PVAV(0x23a168) at 0x182991c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)

>perl -MDevel::Size=total_size -MDevel::Peek -e"my @a; $#a if $ARGV[0]
+; Dump \@a,1" 1
SV = RV(0x239340) at 0x239334
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182991c
  SV = PVAV(0x23a168) at 0x182991c
    REFCNT = 2
    FLAGS = (PADMY,RMG)
    MAGIC = 0x1823de4
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x239254
      SV = PVMG(0x18fb364) at 0x239254
        REFCNT = 1
        FLAGS = (GMG,SMG)
        IV = 0
        NV = 0
        PV = 0
        MAGIC = 0x1823f0c
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x182991c
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x239254
    FLAGS = (REAL)
[download]

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