#!/usr/bin/perl -w
use strict;

my $infile = $ARGV[0];
open INPUT, "<", $infile
  or die "Couldn't open '$infile' : $!";
my $input = do { local $/; <INPUT> };

my %histogram;

for my $char (split //, $input) {
  $histogram{$char}++;
};

for (keys %histogram) {
  print "$_ occurred $histogram{$_} times.\n";
};