Better yet:

use feature 'say';

my $output = "data.txt";
open(DAT, '+<', $output) || die("Cannot Open File");

my $line;
$line = <DAT> until eof DAT;
my ($id) = $line =~ m/\A(\d+):/;

say DAT join ':', ++$id, $name, $number, $address, $phone, $email;

close(DAT);
[download]

This way, the regular expression is only executed once - on the last line.

Sorry for the double post, I can't edit my comments (not enough powers yet).