#!/usr/bin/perl
use strict;
use warnings;

my $file = $ARGV[0] || 'foo.txt';
open (INPUT, '<', $file) or die "Unable to open $file for reading : $!";

my %word;
while ( <INPUT> ) {
    chomp;
    $word{$_}++ for split " ";
}
print "$_ : $word{$_}\n" for sort { $word{$b} <=> $word{$a} } keys %word;