<IN>;    # skip "--start of file--"
while(  <IN>  ) {
    if(  $_ eq "#:lav\n"  ) {
        local $/= "#:eof\n";
        my $block= <IN>;
        # ... process $block here ...
    } elsif(  $_ eq /^--end of file--/  ) {
        warn "Unexpected line: $_";
    }
}