my @lines;
$#lines = 12_000_000;
@lines = ();                 <------ was missing
push @lines, $_ while <IN>;

##</code><code>##

$ perl -e'print "abcdef\n" for 1..11_000_000' |
   perl -E'
      $#a=11_000_000;
      @a=();
      push @a, $_ while <>;
      say int(`ps --no-heading -o vsz $$`/1000)
'
480