For fun, I wrote one.

package Data::CircularBuffer;

use strict;
use warnings;

# This does nothing if 'use threads;' wasn't previously executed.
use threads::shared qw( bless );

sub new {
   my ($class, $n) = @_;
   ++$n;  # Used to differentiate full from empty.
   return bless({
      size => $n,
      buf  => [ (undef) x $n ],
      head => 0,
      tail => 0,
   }, $class);
}

sub is_empty {
   my ($self) = @_;
   return $self->{head} == $self->{tail};
}

# Returns empty list if the buffer is empty. That means the
# following will work even if the buffer contains false elements:
#    while (my ($element) = $cb->shift()) { ... }
sub shift {
   my ($self) = @_;
   return () if $self->is_empty();

   my $val = $self->{buf}[ $self->{head} ];
   $self->{buf}[ $self->{head} ] = undef;
   $self->{head} = ( $self->{head} + 1 ) % $self->{size};
   return $val;
   
}

# Returns number of items successfully inserted.
sub push {
   my $self = shift;

   my $inserted = 0;
   for (@_) {
      my $new_tail = ( $self->{tail} + 1 ) % $self->{size};
      last if $new_tail == $self->{head};

      $self->{buf}[ $self->{tail} ] = $_;
      $self->{tail} = $new_tail;
   }

   return $inserted;
}

1;
[download]

Untested. Should work with a writer in one thread and a reader in another thread without locks.

Update: Now clears elements that have been shifted out of the buffer to ensure timely destruction.