arabic digit indicates instance of a char'
"x" indicates no repetition/dupe  (6); 
and roman numerals indicate distinct repeated chars (VI) AT THE FIRST REPEAT

my $str = "PERL MONKS ROCKS!!!";
P  1      x
E  1      x
R  1            I
L  1      x
   1            repeated later
M  1      x
O  1            repeated later
N  1      x
K  1            repeated later
S  1            repeated later
   2            II
R  2            I
O  2            III
C  1      X
K  2            IV
S  2            V
!  1            VI
!  2            VI
!  3            VI