Here's my hacky solution that works:

use strict;
my @array = (
        "1.found",
        "2.found", 
        "4.found", 
        "5.found",
        "6.found",
        "8.found",
);
  
my $counter = substr($array[0], 0, 1);

for (@array) {
        my ($num) = /^(\d+)/;
        if ($counter < $num) {
                print "$counter.lost\n";
                $counter = $counter + 2;
        } else {
                $counter++;
        }
}
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