How big are these attachments? It looks like you're putting the entire content of <STDIN> to $str, and this might be causing the seg faults. I think the following (untested) code does what the code does, without consuming too much memory. Comments welcome.

while (<>) {
    my $val='';
    if (/^\#:lav/) {
        $_.=<>;
        $val.="$_\n";
        while (/^#:eof/m) {
            $val.="$_\n";
            $_.=<>;
        }
    }
    print $val;
}
[download]

Update: fixed the bonehead error of using next inside the loops.