open my $fd, '<', 'file' or die "open: $!";
my @all;
while (<$fd>) {
  chomp;
  push @all, split /,/;
}
close $fd;
@all = sort { $a <=> $b } @all;
my @result;
for (my $i = 0; $i < @all; $i+=2) {
  push @result, "$all[$i]," . (
    defined $all[$i+1] ? $all[$i+1] : ''
  );
}
print "$_\n" for @result;