first point is, you only have letters, no numbers, so Perl won't "increment" them

Really?

$ perl -le '$c = "a"; print ++$c'
b
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Check perlop:

The auto-increment operator has a little extra builtin magic to it. ... 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 ...

Update: Though you're actually right, but only by coincidence. Using the postfix autoincrement won't make the hash elements incremented.

$ perl -Mstrict -MData::Dumper -we 'my @a = qw(a b c d); \
  my %h = map { $_++ } @a; print Dumper \%h'
$VAR1 = {
          'c' => 'd',
          'a' => 'b'
        };
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s**lil*;  $*=join'',sort split q**;  s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)
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