#!/usr/bin/perl
use warnings;
use strict;
while (my $s = <DATA>) {
    chomp $s;
    my $d = () = $s =~ /(?:(?:^|,)[^,])/g; 
    print "$d\n";
}
__DATA__
1,2,3,4,5
,1,2,3,4
1,2,3,4,,
,,,,,,1,2,3,,,,,4,,,,,,,,