Use keys in scalar context:

my $count = keys %{ $testHash{$year} }
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It might be better not to output the comma, though. Just invert the logic and print it before each sequence except the first one which is easy to detect:

my $first = 1;
while (...) {
    print ',' unless $first;
    undef $first;
    print "..."; # print the sentence
}
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