When you are counting in base-10 and you reach 99, you add a leading "1" instead of wrapping back to only "00". The magic autoincrement does the same with letters: when you reach the end of the alphabet, it adds a leading "a" while wrapping back instead of repeating a previous value.

This is also why "z" becomes "aa":

$ perl -e '$X = "z"; print $X, "\n"; $X++; print $X, "\n";'
z
aa

In reply to Re^3: How the auto-increment operator works? by jcb
in thread How the auto-increment operator works? by zapdos

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