Well, I cant see anything obviously wrong with this, except that it uses unobvious methods to accomplish its task and is therefore difficult to understand. If I understand you right this is the opposite of the 'rangification' problem that pops up in various places on a regular basis. In that problem one wishes to take a set of numbers and convert them into a format of begin-end. Your problem (if I understand it correctly) is even easier. You wish to go through a list of numbers to find missing elements in the list. So as I couldnt see what was wrong I decided Id write a quick sub to do the same thing but is a bit more obvious (and less of a memory hog, and presumably faster to boot :-) Note that there is no need for the hash nor the grep.

sub find_holes {
    my $list = shift;
    my @list = sort { $a <=> $b } @$list;

    my $last = $list[0];
    my $l    = length($last);
    my @vacancies;

    foreach my $v (@list) {
        #If the difference is larger than 1 then theres a hole
        if ( $v - $last > 1 ) {
            push @vacancies, sprintf( "%0${l}d", $_ ) #store whats mis
+sing
                for ( $last + 1 .. $v - 1 );
        }
        $last = $v; #keep track of the last number we saw
    }
    return \@vacancies; # return the omitted elements
}

print join "\n", @{find_holes( [ '00001', '00002', '00005', '00006',
                                 '00009', '00010', '00013', '00025' ] 
+)};
__END__
(edited) Output:
00003 00004 00007 00008 00011 00012 00014 00015 00016 
00017 00018 00019 00020 00021 00022 00023 00024
[download]

Apologies if Ive completely missed the point of your routine.

HTH

Yves / DeMerphq
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