For when, with the dawn of a new day, your mind is more clear:

my $n = scalar @f;

scalar @f gives the number of elements of the array, which is probably not what you want if you're iterating over the elements of the array:

c:\@Work\Perl>perl -wMstrict -le
"my @f = (9, 8, 7);
 my $n = scalar @f;
 for (0 .. $n) {
   print qq{index $_: element $f[$_]};
   }
"
index 0: element 9
index 1: element 8
index 2: element 7
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
index 3: element
[download]

Better to use  $#f which yields the highest index of the array. Then,
    my $n = $#f;
would give you something sensible, and you don't even really need the intermediate  $n variable:

c:\@Work\Perl>perl -wMstrict -le
"my @f = (9, 8, 7);
 for (0 .. $#f) {
   print qq{index $_: element $f[$_]};
   }
"
index 0: element 9
index 1: element 8
index 2: element 7
[download]