#!/usr/local/bin/perl
#
use strict;
use warnings;

print
   map { s{0{2,}}{0}; $_ }
   <DATA>;

__END__
215000007801
300000324002
890000457651
210004563401
201045139158

##</code><code>##

21507801
30324002
890457651
2104563401
201045139158