You can leave off the explicit scalar if you are evaluating keys in a scalar context, as in:

my $count = keys %class1;
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Regarding your second question, yes you can merge hashes together in a simple list. Perl transforms each hash into a list of key-value pairs, and the last value for any given key in the list is what is set in %merged:

my %merged = (%class1, %class2, %class3);
my $count = keys %merged;
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This version is somewhat more readable than your original one, at the expense of a little inefficiency.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print