Programming Perl, 4th Edition

Autoincrement and Autodecrement

The ++ and –– operators work as in C. That is, when placed before a variable, they increment or decrement the variable before returning the value; when placed after, they increment or decrement the variable after returning the value. For example, $a++ increments the value of scalar variable $a, returning the value it had before the increment. Similarly, ––$b{(/(\w+)/)[0]} decrements the element of the hash %b indexed by the first “word” in the default search variable ($_) and returns the value after the decrement.^[60] Note that just as in C, Perl doesn’t define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behavior. Avoid statements like:

$i = $i++;
print ++$i + $i++;

Perl will not guarantee the results of such code.

The autoincrement operator has a little extra built-in magic. 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 only in string contexts since it was set, has a value that is not the null 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:

my $foo; $foo = "99"; print ++$foo; # prints "100" $foo = "a9"; print ++$foo; # prints "b0" $foo = "Az"; print ++$foo; # prints "Ba" ...