$text = <<'EOT';
Message-ID: <ODM2bWFpbGVyLmRpZWJlYS40MjYyNjE2LjEyOTU1NDE2MTg=@out-p-h.customernews.net>
From: "GenericOnline Pharmacy" <marmot@furrytorium.com>
To: "Angie Morestead" <marmot@furrytorium.com>
Subject: Buy drugs online now!
Date: Thu, 20 Jan 2011 18:40:18 +0200
Content-Type: multipart/related; boundary="----=_Weigard_drugs_CG_0"
EOT

$text =~ /^Subject:.+drugs/m; # Anchor just after \n, before Subject. 
                              # Matches 'Subject: Buy drugs'

$text =~ /\nSubject:.+drugs/; # Equivalent

$text =~ /^Subject:.+drugs/ms;  # '.' matches newlines, all the way to
                                # '..._Weigard_drugs', which is not what we wanted.

$text =~ /^Subject:.+?drugs/ms; # '.' matches newlines, but searches from current string
                                # position, stopping when it matches 'Subject: Buy drugs'.
                                # This is a little slower than the first two, but
                                # equivalent. /s is countered by the .+?, but if 'drugs'
                                # was not in the Subject line, the regex would keep keep
                                # on going.

# Here are some fun ones.  

# The email address should be "Furry Marmot" <marmot@furrytorium.com>, or just
# marmot@furrytorium.com. Anything else is spam. 
print "Spam!!!\n" 
    if $text =~ /^(?:From|To):\s*"(?!.+Furry Marmot)[^"]*" <marmot\@furrytorium\.com>/m;

# Regarding the [^"]*, if the regex finds Furry Marmot in quotes, it fails and this isn't
# spam. But if it finds something else, we still have to match something between the
# quotes, and then match the email to determine if it is spam.

# I should never see anything from me, to me. 

print "Spam!!!\n" if 
    $text =~ /(?=^From:[^\n]+marmot\@furrytorium\.com).+^To:[^\n]+marmot\@furrytorium\.com/ms;

# This starts at the beginning of header block, finds From: line with my email address,
# resets to start of block (because of zero-width lookahead assertion), then finds To:
# line with my email address. It is the equivalent of...

if ($text =~ /^From:.+marmot\@furrytorium\.com)/m && /^To:.+marmot\@furrytorium\.com/m) {
    print "Spam!!!\n" 
}

# ...but I can include the single pattern in a list of patterns that I might want to match
# against the string.