use strict;
use warnings;

open( INPUT, "sort $ARGV[0] |" ) or die "can't sort $ARGV[0]: $!";

my $outname = "";
while (<INPUT>) {
    if ( /^(.+?),/ ) {
        my $newname = $1;
        if ( $newname ne $outname ) {
            close OUT if ( $outname );
            open( OUT, ">$newname" ) or die "can't output to $newname:$!";
            $outname = $newname;
        }
        print OUT;
    }
    else {
        warn "Sorted input from $ARGV[0] had unusable data at line $.: $_\n"
    }
}
close OUT;