You could do

+    my $keyword = 0;
+    my $usage   = 0;
     foreach my $word ( @_ ) {
         if ($word eq 'FATAL') {
+            $usage ||= $keyword;
+            $keyword = 1;
             $fatal = 1;
             $no_fatal = 0;
         }
         elsif ($word eq 'NONFATAL') {
+            $usage ||= $keyword;
+            $keyword = 1;
             $fatal = 0;
             $no_fatal = 1;
         }
         elsif ($catmask = $Bits{$word}) {
+            $keyword = 0;
             $mask |= $catmask ;
             $mask |= $DeadBits{$word} if $fatal ;
             $mask &= ~($DeadBits{$word}|$All) if $no_fatal ;
         }
         else
           { Croaker("Unknown warnings category '$word'")}
     }
+    warn("No warning categories specified for FATAL or NONFATAL")
+        if $usage;
[download]

but it breaks backwards compatibility if someone does

use warnings FATAL => @fatals;
[download]

As such, this might be better off as a Perl::Critic check.