此代码演示

use strict;
use warnings;

my $str = '> plantagenetgoosewagonattributes';

if ($str =~ />(.*)[^a]+/) {
  my $substr = $1;
  my %counts;
  $counts{$_}++ for $substr =~ /./g;
  print "'$_' - $counts{$_}\n" for sort keys %counts;
}

产量

' ' - 1
'a' - 4
'b' - 1
'e' - 4
'g' - 3
'i' - 1
'l' - 1
'n' - 3
'o' - 3
'p' - 1
'r' - 1
's' - 1
't' - 5
'u' - 1
'w' - 1