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    $/ = '';
    $header = <MSG>;
    $header =~ s/\n\s+/ /g;	 # merge continuation lines
    %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );

That solution doesn't do well if, for example, you're trying to
maintain all the Received lines.  A more complete approach is to use
the Mail::Header module from CPAN (part of the MailTools package).

=head2 How do I decode a CGI form?

You use a standard module, probably CGI.pm.  Under no circumstances
should you attempt to do so by hand!

You'll see a lot of CGI programs that blindly read from STDIN the number
of bytes equal to CONTENT_LENGTH for POSTs, or grab QUERY_STRING for
decoding GETs.  These programs are very poorly written.  They only work
sometimes.  They typically forget to check the return value of the read()
system call, which is a cardinal sin.  They don't handle HEAD requests.
They don't handle multipart forms used for file uploads.  They don't deal
with GET/POST combinations where query fields are in more than one place.
They don't deal with keywords in the query string.

In short, they're bad hacks.  Resist them at all costs.  Please do not be
tempted to reinvent the wheel.  Instead, use the CGI.pm or CGI_Lite.pm
(available from CPAN), or if you're trapped in the module-free land
of perl1 .. perl4, you might look into cgi-lib.pl (available from
http://www.bio.cam.ac.uk/web/form.html).

Make sure you know whether to use a GET or a POST in your form.
GETs should only be used for something that doesn't update the server.
Otherwise you can get mangled databases and repeated feedback mail
messages.  The fancy word for this is ``idempotency''.  This simply
means that there should be no difference between making a GET request
for a particular URL once or multiple times.  This is because the
HTTP protocol definition says that a GET request may be cached by the
browser, or server, or an intervening proxy.  POST requests cannot be
cached, because each request is independent and matters.  Typically,
POST requests change or depend on state on the server (query or update
a database, send mail, or purchase a computer).

=head2 How do I check a valid mail address?

You can't, at least, not in real time.  Bummer, eh?

Without sending mail to the address and seeing whether there's a human
on the other hand to answer you, you cannot determine whether a mail
address is valid.  Even if you apply the mail header standard, you
can have problems, because there are deliverable addresses that aren't
RFC-822 (the mail header standard) compliant, and addresses that aren't
deliverable which are compliant.

Many are tempted to try to eliminate many frequently-invalid
mail addresses with a simple regexp, such as
C</^[\w.-]+\@([\w.-]\.)+\w+$/>.  It's a very bad idea.  However,
this also throws out many valid ones, and says nothing about
potential deliverability, so is not suggested.  Instead, see
http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
which actually checks against the full RFC spec (except for nested
comments), looks for addresses you may not wish to accept mail to
(say, Bill Clinton or your postmaster), and then makes sure that the
hostname given can be looked up in the DNS MX records.  It's not fast,
but it works for what it tries to do.

Our best advice for verifying a person's mail address is to have them
enter their address twice, just as you normally do to change a password.
This usually weeds out typos.  If both versions match, send
mail to that address with a personal message that looks somewhat like:

    Dear [e-mail],

    Please confirm the mail address you gave us Wed May  6 09:38:41
    MDT 1998 by replying to this message.  Include the string
    "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
    start with "Nik...".  Once this is done, your confirmed address will
    be entered into our records.

If you get the message back and they've followed your directions,
you can be reasonably assured that it's real.

A related strategy that's less open to forgery is to give them a PIN
(personal ID number).  Record the address and PIN (best that it be a
random one) for later processing.  In the mail you send, ask them to
include the PIN in their reply.  But if it bounces, or the message is
included via a ``vacation'' script, it'll be there anyway.  So it's
best to ask them to mail back a slight alteration of the PIN, such as
with the characters reversed, one added or subtracted to each digit, etc.

=head2 How do I decode a MIME/BASE64 string?

The MIME-tools package (available from CPAN) handles this and a lot
more.  Decoding BASE64 becomes as simple as:

    use MIME::base64;
    $decoded = decode_base64($encoded);

A more direct approach is to use the unpack() function's "u"
format after minor transliterations:

    tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
    tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
    $len = pack("c", 32 + 0.75*length);   # compute length byte
    print unpack("u", $len . $_);         # uudecode and print

=head2 How do I return the user's mail address?

On systems that support getpwuid, the $E<lt> 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 mail address can mean that this generates addresses
that the company's mail system will not accept, so you should ask for
users' mail 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.

=head2 How do I send mail?

Use the C<sendmail> program directly:

    open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
                        or die "Can't fork for sendmail: $!\n";
    print SENDMAIL <<"EOF";
    From: User Originating Mail <me\@host>
    To: Final Destination <you\@otherhost>
    Subject: A relevant subject line

    Body of the message goes here, in as many lines as you like.
    EOF
    close(SENDMAIL)     or warn "sendmail didn't close nicely";

The B<-oi> option prevents sendmail from interpreting a line consisting
of a single dot as "end of message".  The B<-t> option says to use the
headers to decide who to send the message to, and B<-odq> says to put
the message into the queue.  This last option means your message won't
be immediately delivered, so leave it out if you want immediate
delivery.

Or use the CPAN module Mail::Mailer:

    use Mail::Mailer;

    $mailer = Mail::Mailer->new();
    $mailer->open({ From    => $from_address,
                    To      => $to_address,
                    Subject => $subject,
                  })
        or die "Can't open: $!\n";
    print $mailer $body;
    $mailer->close();

The Mail::Internet module uses Net::SMTP which is less Unix-centric than
Mail::Mailer, but less reliable.  Avoid raw SMTP commands.  There
are many reasons to use a mail transport agent like sendmail.  These 
include queueing, MX records, and security.

=head2 How do I read mail?

Use the Mail::Folder module from CPAN
(part of the MailFolder package) or the Mail::Internet module from
CPAN (also part of the MailTools package).

   # sending mail
    use Mail::Internet;
    use Mail::Header;
    # say which mail host to use
    $ENV{SMTPHOSTS} = 'mail.frii.com';
    # create headers
    $header = new Mail::Header;
    $header->add('From', '[e-mail]');
    $header->add('Subject', 'Testing');
    $header->add('To', '[e-mail]');
    # create body
    $body = 'This is a test, ignore';
    # create mail object
    $mail = new Mail::Internet(undef, Header => $header, Body => \[$body]);
    # send it
    $mail->smtpsend or die;

Often a module is overkill, though.  Here's a mail sorter.

    #!/usr/bin/perl 
    # bysub1 - simple sort by subject
    my(@msgs, @sub);
    my $msgno = -1;
    $/ = '';                    # paragraph reads
    while (<>) {
        if (/^From/m) {
            /^Subject:\s*(?:Re:\s*)*(.*)/mi;
            $sub[++$msgno] = lc($1) || '';
        }
        $msgs[$msgno] .= $_;
    } 
    for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
        print $msgs[$i];
    }

Or more succinctly, 

    #!/usr/bin/perl -n00
    # bysub2 - awkish sort-by-subject
    BEGIN { $msgno = -1 }
    $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
    $msg[$msgno] .= $_;
    END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }

=head2 How do I find out my hostname/domainname/IP address?

The normal way to find your own hostname is to call the C<`hostname`>
program.  While sometimes expedient, this has some problems, such as
not knowing whether you've got the canonical name or not.  It's one of
those tradeoffs of convenience versus portability.

The Sys::Hostname module (part of the standard perl distribution) will
give you the hostname after which you can find out the IP address
(assuming you have working DNS) with a gethostbyname() call.

    use Socket;
    use Sys::Hostname;
    my $host = hostname();
    my $addr = inet_ntoa(scalar(gethostbyname($name)) || 'localhost');

Probably the simplest way to learn your DNS domain name is to grok
it out of /etc/resolv.conf, at least under Unix.  Of course, this
assumes several things about your resolv.conf configuration, including
that it exists.

(We still need a good DNS domain name-learning method for non-Unix
systems.)

=head2 How do I fetch a news article or the active newsgroups?

Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
This can make tasks like fetching the newsgroup list as simple as:

    perl -MNews::NNTPClient
      -e 'print News::NNTPClient->new->list("newsgroups")'

=head2 How do I fetch/put an FTP file?

LWP::Simple (available from CPAN) can fetch but not put.  Net::FTP (also
available from CPAN) is more complex but can put as well as fetch.

=head2 How can I do RPC in Perl?

A DCE::RPC module is being developed (but is not yet available), and
will be released as part of the DCE-Perl package (available from
CPAN).  No ONC::RPC module is known.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
All rights reserved.

When included as part of the Standard Version of Perl, or as part of
its complete documentation whether printed or otherwise, this work
may be distributed only under the terms of Perl's Artistic License.
Any distribution of this file or derivatives thereof I<outside>
of that package require that special arrangements be made with
copyright holder.

Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.