... as explained in perlop#Auto-increment-and-Auto-decrement:

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"

[download]

Cf. First JAPH - Spell perl in two hundred and eighty five thousand and seventy four easy steps. ;-)

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

In reply to Re^2: POE - can't increment within sub by Athanasius
in thread POE - can't increment within sub by ljamison

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