There is nothing special about sending email from mod_perl, it's just that we do it a lot. There are a few important issues. The most widely used approach is starting a sendmail process and piping the headers and the body to it. The problem is that sendmail is a very heavy process, and it makes mod_perl processes less efficient.
If you don't want your process to wait until delivery is complete, you can tell sendmail not to deliver the email straight away, but to either do it in the background or just queue the job until the next queue run. This can significantly reduce the delay for the mod_perl process, which would otherwise have to wait for the sendmail process to complete. You can specify this for all deliveries in sendmail.cf, or for individual email messages on each invocation on the sendmail command line. Here are the options:
Deliver in the background
Queue, and also defer the DNS/NIS lookups
The current trend is to move away from sendmail and switch to using lighter mail delivery programs such as qmail or postfix. You should check the manpage of your favorite mailer application for equivalents to the configuration presented for sendmail.
Alternatively, you may want to use Net::SMTP to send your mail without calling an extra process. The main disadvantage of using Net::SMTP is that it might fail to deliver the mail because the destination peer server might be down. It can also be very slow, in which case the mod_perl application will do nothing while it waits for the mail to be sent.