Not as groovy as merlyn's post, but I'd already put this together so here it is anyway

my @numbers  = ('25', '12', '32','56','45','21','65');
my @needed   = ('25','32','45','65');
my @flipflop = map [$needed[$_], $needed[$_ + 1]],
               grep { $_ % 2 == 0 } 0 .. $#needed;

for(@numbers) {
  print "$_ " and next
    if /\A$flipflop[0]->[0]\z/ .. /\A$flipflop[0]->[1]\z/;
  shift @flipflop and print "\n";
}

__output__

25 12 32 
45 21 65
[download]

See. map, grep and perlop for more info.
HTH

_________
broquaint