One way:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @ra = qw(12 -3 4 71 -11 -598 -100203);
 ;;
 my $i_min;
 ;;
 $ra[$_] >= 0 and
 (! defined($i_min) or $ra[$_] < $ra[$i_min]) and
 $i_min = $_ for 0 .. $#ra;
 ;;
 if (defined $i_min) {
   print qq{i of min lwn ($ra[$i_min]) == $i_min};
   }
 else {
   print 'no lowest whole number in array';
   }
"
i of min lwn (4) == 2
[download]

(There may be specific utilities for this purpose in List::Util or the older List::MoreUtils.)

Update: This approach returns the lowest index of more than one LWN in an array. To obtain the highest index of multiple LWNs, change the  < (less than) comparison in the
    (! defined($i_min) or $ra[$_] < $ra[$i_min])
term to  <= (less than or equal):
    (! defined($i_min) or $ra[$_] <= $ra[$i_min])