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

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