Any idea how to stop this?

Several ideas, most of which you wouldn't like. However, the implication is that $line sometimes doesn't contain the $j field. You could provide a default value for it if that is appropriate, or take some other action such as ignoring the line and writing an error to your error log.

The important point is that Perl is telling you that you haven't handled an unexpected condition and that you need to take appropriate action.

As an aside, your variable names are awful and your indentation is pretty bad too! Note that $a and $b are special variables and their use outside of a sort block will sometimes bite you on the bum.

$d."|".$g."|".$j."|".$k."|".$e.", ".$f."\n" is better written: "$d|$g|$j|$k|$e, $f\n".