Counting is probably the best way, even though it isn't exciting. Of course there are other ways. One that comes to mind is:

my %h;
@h{@a} = ();  # make a set of the unique strings

for my $k ( keys %h ) {
  $h{$k} = () = grep { $_ eq $k } @a; # get count of matching elements
}
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But somehow I doubt that's more efficient. ;-)