MIME::Parser can do all of that for you. It's parse() method will automatically break apart the message and save the bits in separate files.

#!/usr/bin/perl

use MIME::Parser;

my $p = MIME::Parser->new;

$p->output_dir( "/Users/brian/Desktop" );

$p->parse( \*STDIN );
[download]

If you want the whole message, read it and store it in a variable then use the parse_data() method instead.

I have a full program in my "Detaching attachments" article in the September 2004 issue of The Perl Journal.