To get the number of matches you evaluate grep in scalar context. Here:

#!/usr/bin/perl

use strict;
use warnings;

my @items = qw/yes no no no yes yes no no/;
my $number_of_matches = grep(/yes/,@items);

print "$number_of_matches\n";
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Prints 3.

Update: Noticed that I left out @items, and I see I am not alone in my confusion about exactly what it was the OP wanted.