This is not a bug nor a trick neither, the ++ operator works on strings stored in scalar variables as long as they have never been used in a numeric context, once these variables are used in a numeric context a re-initialization of the variable's value into a number takes place..

#!/usr/local/bin/perl
$c = "c";
print ++$c," from c\n";
print ++$c," from d\n";
#apply the same in numeric context
$c+=0;
print $c, " from numeric operation\n";
print ++$c, " incremented after numeric assignment";
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However, Perl would throw a warning when it reaches to "$c+=0;" telling you that a non-numeric argument is used in the addition operation if you have turned warnings on via "use warnings" which can save you from introducing a potentially hard-to-track bug otherwise...