#! perl -slw
use strict;
use Data::Dump qw[ pp ];


my @freq;
my @data = qw[ AABBC BAABC AABBD AACBB ];

for my $s ( @data ) {
    ++$freq[ $_ ]{ substr $s, $_, 1 } for 0 .. length( $s ) -1;
}

pp \@freq;

for my $pos ( @freq ) {
    ( $pos->{ $_ } //= 0 ) /= 4 for 'A' .. 'D';
}

pp \@freq;

__DATA__
C:\test>1162755
[
  { A => 3, B => 1 },
  { A => 4 },
  { A => 1, B => 2, C => 1 },
  { B => 4 },
  { B => 1, C => 2, D => 1 },
]

[
  { A => 0.75, B => 0.25, C => 0, D => 0 },
  { A => 1, B => 0, C => 0, D => 0 },
  { A => 0.25, B => 0.5, C => 0.25, D => 0 },
  { A => 0, B => 1, C => 0, D => 0 },
  { A => 0, B => 0.25, C => 0.5, D => 0.25 },
]