So, the ++ operator has highest precedence. According to perlop it returns the value first then autoincrements it. Then $a is evlauated, then print. Step by step it looks like this:

print $a, $a++;
print $a, 'A';  #++returns $a prior to incrementing
print 'B', 'A'; #$a is evaluated to it's after increment value of 'B'
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