sub max2 { $_[0] > $_[1] ? $_[0] : $_[1] }

my $filename = ...;

my %ignore = map { lc($_) => 1 } qw(
   a
   an
   are
   i
   if
   in
   is
   it
   m
   on
   re
   s
   to
   the
   ...
);

open(my $fh_in, '<', $filename)
  or die("Unable to open input file: $!\n");

my %counts;

while (<$fh_in>) {
   $_ = lc($_);
   while (/([a-z]+)/g) {
      next if $ignore{$1};
      ++$counts{$1};
   }
}

my @words_ordered = sort { $counts{$b} <=> $counts{$a} } keys %counts;

foreach (0 .. max2(99, $#words_ordered)) {
   print("Word $_ was found $counts{$_} times.\n");
}