You should first start by using the right kind of quotes. @ is going to interpolate within double quotes, as the sigil of an array. You need single quotes that don't trigger interpolation. Then, you could use the substitution operator like this:

my $email = 'dummy@dumdum.com';

$email =~ s/\@/\\@/;

print $email, "\n";
[download]

...could... but there's another way too. quotemeta will "escape" any nonword character. While this does include @, it also includes . (dot). That may or may not be a problem for you. You'll have to decide.

Further reading: Interpolation within quotes is discussed in excellent detail in perlop. The substitution operator is also discussed in perlop, as well as perlre and perlretut.