while (<REG>) {
    chomp;
    my($key, $data) = split "\t", $_, 2;
    push @{$R{$key}}, $data;
}

##</code><code>##

foreach my $key (keys %R) {
    open my $fh, '>', "$key.out" or die "Cannot open file $key.out: $!";
    foreach my $row (@{$R{$key}}) {
        print $fh "$key\t$row\n";
    }
}