If you want to do your matching about the whole string (containing multiple lines), then you want /s (also let the dot meta character match line breaks).

my ($body) = $line =~ /^body: \n (.*) \n attachment/xsi;
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Or you are matching line by line, then you should have a look at the flip-flop operator in perlop.

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