use vars qw($big_array);

sub get_big_array
{
  if (!$big_array)
  {
    $big_array = [ ... construct big array... ];
  }
  return $big_array;
}

sub clear_big_array
{
  undef $big_array;
}

sub processA {
  common_stuff($_);
  # special stuff
}

sub processB {
  loop (1..1000) {
    common_stuff($_);
    # special stuff
  }
}

sub common_stuff
{
  my $ba = get_big_array();
  ...
}