UPDATE:
I actually read the problem wrong, and that was answered by a lot of people by now... so ignore this unless you want to count the nucleotide amounts :)

Heres a code, that requires the user to give a filename in commandline. You should be able to use the sub within your own code as well.

#!/usr/bin/perl

use warnings;
use strict;

my $inf=shift @ARGV || or die "Need to give a input file.\n";
open (INF,shift @ARGV);

sub print_map (\$\$) {
  my $name_r=shift @_;
  my $code_r=shift @_;

  my %hash;
  print "$$name_r\n";
  while ($$code_r=~m/(\w)/g) { $hash{$1}++;}
  foreach (sort {$a cmp $b} keys %hash) {
    print "$_: $hash{$_}\n";
  }
}

my ($code,$name);
while (<INF>) {
  chomp;
  if (m/^>(.+)/) {
    print_map ($name,$code) if ($name);
    $name=$1;
  } else {
    $code.=uc($_);
  }
}
print_map($name,$code);

exit;
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