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 missing
				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