#! perl
use strict;
use warnings;
use Data::Dump;
use List::Util 'max';

# 1. Configuration

use constant UNIT => '* ';
my  @required_keys = qw(5 a foo);

# 2. Read in and count the data

my %counts = map { $_ => 0 } @required_keys;

while (<DATA>)
{
    ++$counts{$_} for split;
}

dd \%counts;    # Verify hash contents

# 3. Generate the histogram

print "\nHistogram:\n";
my $max_len = max map { length } keys %counts;

for (sort keys %counts)
{
    printf "%*s: ", $max_len, $_;
    print  UNIT for 1 .. $counts{$_};
    print  "\n";
}

__DATA__
a -2 3 b 0xffff c 2 b a 4 a a 200
0xffff 17 a a c 3 200 201 b -2 b
a b c a a 2 c -2