That backslash is not required.

#!perl

use strict;
use warnings;

my $email = 'peter@gmail.com';
my ($u,$d) = split (/@/,$email);
print "$email => $u,$d\n";
[download]

outputs:

peter@gmail.com => peter,gmail.com
[download]

In other words, if $email properly contains the email address, the split command that the original poster showed works as they wanted it to -- without your unneeded backslash (though your backslash doesn't change the results, in this case). Hence, the earlier monks were properly digging into the actual contents of $email to try to debug the problem.