Not perl, but from the procmail manpage:

If the regular expression contains ‘^TO_’ it will be substituted by ‘(^((Original-)?(Resent-)?(To |Cc |Bcc) |(X-Envelope |Apparently(-Resent)?)-To) :(.*[^-a-zA-Z0-9_.])?)’, which should catch all destination specifications containing a specific address.

If the regular expression contains ‘^TO’ it will be substituted by ‘(^((Original-)?(Resent-)?(To |Cc |Bcc) |(X-Envelope |Apparently(-Resent)?)-To) :(.*[^a-zA-Z])?)’, which should catch all destination specifications containing a specific word.

If the regular expression contains ‘^FROM_DAEMON’ it will be substituted by ‘(^(Mailing-List : |Precedence :.*(junk |bulk |list) |To : Multiple recipients of |(((Resent-)?(From |Sender) |X-Envelope-From) : |>?From )([^>]*[^(.%@a-z0-9])?(Post(ma?(st(e?r)? |n) |office) |(send)?Mail(er)? |daemon |m(mdf |ajordomo) |n?uucp |LIST(SERV |proc) |NETSERV |o(wner |ps) |r(e(quest |sponse) |oot) |b(ounce |bs\.smtp) |echo |mirror |s(erv(ices? |er) |mtp(error)? |ystem) |A(dmin(istrator)? |MMGR |utoanswer))(([^).! :a-z0-9][-_a-z0-9]*)?[%@>\t ][^<)]*(\(.*\).*)?)?$([^>] |$)))’, which should catch mails coming from most daemons (how’s that for a regular expression :-).

If the regular expression contains ‘^FROM_MAILER’ it will be substituted by ‘(^(((Resent-)?(From |Sender) |X-Envelope-From) : |>?From )([^>]*[^(.%@a-z0-9])?(Post(ma(st(er)? |n) |office) |(send)?Mail(er)? |daemon |mmdf |n?uucp |ops |r(esponse |oot) |(bbs\.)?smtp(error)? |s(erv(ices? |er) |ystem) |A(dmin(istrator)? |MMGR))(([^).! :a-z0-9][-_a-z0-9]*)?[%@>\t ][^<)]*(\(.*\).*)?)?$([^>] |$))’ (a stripped down version of ‘^FROM_DAEMON’), which should catch mails coming from most mailer-daemons.