my @stuff = 0 .. 9;
my @lastitem ;
for my $item ( @stuff ){
  my $next = $item;
  if( @lastitem and  $item eq $lastitem[-1] ){
    print "they measure up\n";
  }
  push @lastitem, $item;
  shift @lastitem while 3 == @lastitem; ### keep at most 3 last items
}