How do I return the user's email address?

On systems that support getpwuid, the $< variable and the Sys::Hostname module (which is part of the standard perl distribution), you can probably try using something like this: 

    use Sys::Hostname;
    $address = sprintf('%s@%s', getpwuid($<), hostname);

Company policies on email address can mean that this generates addresses that the company's email system will not accept, so you should ask for users' email addresses when this matters. Furthermore, not all systems on which Perl runs are so forthcoming with this information as is Unix.

The Mail::Util module from CPAN (part of the MailTools package) provides a mailaddress function that tries to guess the mail address of the user. It makes a more intelligent guess than the code above, using information given when the module was installed, but it could still be incorrect. Again, the best way is often just to ask the user.