Thanks for your suggestion, oko1. If $j=0, the if loop shouldn't run, as the logic condition given in the if statement is if(($energy_rebin[$k] < $energy_before[$j]) && $j!=0){. The problem for this case was that $j was too large due to a typograhical error in the 'for' loop and this yielded the undefined value problem.

However, many thanks for looking through the code and for your suggestion.

Steve