my ($body) = $line =~ /^body: \n (.*) \n attachment/xsi;

##</code><code>##

# this is from perlop
while (<>) {
    $in_header =   1  .. /^$/;
    $in_body   = /^$/ .. eof;
    if ($in_header) {
        # ...
    } else { # in body
        # ...
    }
}