...
if (defined  ($eSheet->{Cells}[$row][0] ))
{
    $master_key=($eSheet->{Cells}[$row][0]->Value);
    $r=$row;
    print "Found (new) master key '$master_key' in row $r\n";
}
else
{
    $master_key=$master_key;
    print "No (new) master key found in row $row, keeping current master key '$master_key' from row $r\n";
}
...
if (defined $eSheet->{Cells}[$row][$col])
{

 $val=($eSheet->{Cells}[$row][$col]->Value);
 print "Found valid value for '$key' in ($row,$col): '$val'\n";

}
else 
{

 $val=$eSheet->{Cells}[$r][$col]->Value;
 print "Found invalid/empty value for '$key' in ($row,$col), reusing the value from ($r,$col): '$val'\n";

}

##</code><code>##

foreach $row( 1 .. $eSheet->{MaxRow} )
{
if (defined  ($eSheet->{Cells}[$row][0] ))
{
$master_key=($eSheet->{Cells}[$row][0]->Value);
$r=$row;
}
else
{
$master_key=$master_key;
}
...

##</code><code>##

foreach $row( 1 .. $eSheet->{MaxRow} )
{
    if (defined  ($eSheet->{Cells}[$row][0] ))
    {
        $master_key=($eSheet->{Cells}[$row][0]->Value);
        $r=$row;
    }
    else
    {
        $master_key=$master_key;
    }