#method 1: Perl will iterate over the array, so we just
#increment the counter at each write
$worksheet->write_string($row, $col++, $_) for @Fld;

#method 2: If you need a loop (so you can do something
#else in the code, too), you can do it like:
for my $item (@Fld) {
    $worksheet->write_string($row, $col++, $item);
    # do something else here...
}

#method 3: If you don't mind destroying the @Fld array,
#you could also:
while (my $item = shift @Fld) {
    $worksheet->write_string($row, $col++, $item);
    # do something else here...
}