#!/usr/bin/perl -w 
use strict;
my %seen;
while (<>) { s![\$\%\@]([a-zA-Z]\w*)!$seen{$1}++!eg  }
my @once = sort grep { $seen{$_} == 1 } keys %seen;
if (@once) {
    print "Possible unused variables found: \n\n", join("\n", @once), "\n";
}

##</code><code>##

$ perl use.pl foo.pl
Possible unused variables found: 

var_a