To solve your case problem, try this:

use strict;
use warnings;

my @words = qw ( a ants cats cats dogs DOGS rabbits r@bbits turtles wa
+rthogs );
my @exclude = qw ( ants Turtles WARTHOGS );

my %exclude = map { lc $_ => '1' } @exclude;

for ( @words ) {
   next if /\W/
        or length $_ < 4
        or $exclude{lc $_}++;
   print "$_\n";
}
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dave