chomp( my @array = <FILE> );

foreach my $previous_line_no ( 0 .. $#array ) { # count through the array.
                                                # runs over its boundary, hmm.
                                                # see with warnings on.
    my $line_no = $previous_line_no + 1;
    if ( $array[$line_no] =~ m/fail/ ) {
        splice( @array, $previous_line_no, 1 ); # take that previous line out.
    }
}