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The auto‐increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern "/^[a−zA−Z]*[0−9]*\z/", the increment is done as a string, preserving each character within its range, with carry:

print ++($foo = "99"); # prints "100"
print ++($foo = "a0"); # prints "a1"
print ++($foo = "Az"); # prints "Ba"
print ++($foo = "zz"); # prints "aaa"

"undef" is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post‐increment of an undef value will return 0 rather than "undef").

The auto‐decrement operator is not magical.

Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live. -- Damian Conway in "Perl Best Practices"
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