#!/usr/bin/perl

open(IN, "<./in.txt");
@lines = <IN>;
close(IN);
@lines = sort { (split(/,/, $b, 2))[0] <=> (split(/,/, $a, 2))[0] } @lines;
open(OUT, ">./out.txt");
print OUT @lines;
close(OUT);

__END__

こういう事？違ってたらごめん