$filename = <$FN[$k]>; is still bad, though. Using glob without looping over its results makes no sense.

use strict;
use warnings;
my @FN = qw( a b );
for my $k (0..$#FN) {
   my $filename = <$FN[$k]>;
   print("$filename\n");
}
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a
Use of uninitialized value in concatenation (.) or string at 689777.pl
+ line 6.
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If @FN contains glob patterns, then he wants

for (@FN) {
   while (defined(my $filename = glob($_))) {
      ...
   }
}
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If @FN contains file names (as it seems to), then he wants

for my $filename (@FN) {
   ...
}
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Although it looks like @FN isn't needed at all.

for (0..18, 20..28, 30..38) {
   my $filename = sprintf('out-02-%02d.txt', $_);
   ...
}
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