Here's how I'd do it

use strict;
use Data::Dumper;

my @nums = map { chomp; $_ } <DATA>;

for(local $_ = 0; $_ < $#nums; $_++) {
  splice @nums, $_, 1 => $nums[$_], ( $nums[$_] + 1 ). ".lost"
    unless $nums[$_] == ( $nums[$_ + 1] - 1 );
}   

print Dumper(\@nums);

__DATA__
1
2
3
6
7
9
10
[download]

And that should spit out the desired results (minus the 'found'). Oh, and looky, a valid use of a C-style for loop!
HTH

_________
broquaint