It does indeed clear the array, but it does not free it the underlying buffer.

Setting $#a extends the visible size of the array, not just the internal one. The elements I proceeded to push onto the array were getting added after the 12 mil empty ones.

@a=(); empties the visible array, but doesn't deallocate the internal buffer like undef @a; does.

$ perl -MDevel::Peek -e'
   my @a = qw( a b c ); Dump(\@a,1);
   @a=();               Dump(\@a,1);
   undef(@a);           Dump(\@a,1);
'
SV = IV(0x816ce54) at 0x816ce58
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x817bc50
  SV = PVAV(0x816d038) at 0x817bc50
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x817b500
    FILL = 2                <---- 3 elements
    MAX = 3                 <---- 4 slots
    ARYLEN = 0x0
    FLAGS = (REAL)
SV = IV(0x816c154) at 0x816c158
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x817bc50
  SV = PVAV(0x816d038) at 0x817bc50
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x817b500
    FILL = -1              <---- 0 elements
    MAX = 3                <---- 4 slots
    ARYLEN = 0x0
    FLAGS = (REAL)
SV = IV(0x816c154) at 0x816c158
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x817bc50
  SV = PVAV(0x816d038) at 0x817bc50
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x0            <---- No array allocated
    FILL = -1              <---- 0 elements
    MAX = -1               <---- 0 slots
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

If you don't want to rely on that, you could do

my @lines;
$#lines = 12_000_000;
$lines[$.-1] = $_ while <>;
$#lines = $.-1;
[download]

Devel::Peek looks interesting. Thanks again.