my $i = 0;
my @spinner = ("|\r", "/\r", "-\r", "\\\r");
while (<>) {   # or any other looping structure
  print STDERR $spinner[$i];
  $i = $i++ % @spinner;
  # do stuff
}

##</code><code>##

sub spinner {
  my $i; my @chars = defined @_ ? @_ : ("|\r", "/\r", "-\r", "\\\r");
  return sub { "$chars[$i++ % @chars]" };
}
my $spinner = spinner();
while (<>) {
  print STDERR $spinner->();
  # do stuff
}