"...I need a count of 'aa' in a string."

If you only want to count the occurance of 'aa' (only two letters - you wrote 'aa') try this:

print scalar grep {$_ eq 'aa'} split /[^a]/, 'dftaatzaaadfaa';

This one counts at two or more occurances of a:

print scalar grep {/aa/} split /[^a]/, 'dftaatzaaadfaa';

Please see also scalar, grep, split and perlretut

Update: Fixed typos

Update 2: Yes, i know - some might say this is yet another "misuse". But it works:

karls-mac-mini:monks karl$ perl -MData::Dump  -E 'dd split /[^a]/, qq(
+dftaatzaaadfaa)';
("", "", "", "aa", "", "aaa", "", "aa")

karls-mac-mini:monks karl$ perl -E 'say scalar grep {$_ eq 'aa'} split
+ /[^a]/, qq(dftaatzaaadfaa)';
2

karls-mac-mini:monks karl$ perl -E 'say scalar grep {/aa/} split /[^a]
+/, qq(dftaatzaaadfaa)';
3
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Regards, Karl