use warnings;
use strict;

my $file = shift;

open my $IN, '<', $file or die "Cannot open '$file' $!";

while ( <$IN> ) {
    my ( $field ) = /^([^,]+)/ or die "Error: field not found.\n";
    open my $OUT, '>>', $field or die "Cannot open '$field' $!";
    print $OUT $_;
    }

close $IN;