my @positive_indices = grep { $arr[$_] > 0 } 0 .. $#arr;
my @negative_indices = grep { $arr[$_] < 0 } 0 .. $#arr;

####

my @positive_indices = map { $arr[$_] > 0 ? $_ : () } 0 .. $#arr;
my @negative_indices = map { $arr[$_] < 0 ? $_ : () } 0 .. $#arr;

####

my @positive_indices;
my @negative_indices;
for (0 .. $#arr)
{
    push @positive_indices, $_ if $arr[$_] > 0;
    push @negative_indices, $_ if $arr[$_] > 0;
}