use strict;
use warnings;
                                                                                
print "Enter expression:  ";
chomp(my $expr = <STDIN>);
                                                                                
my @vars = ($expr =~ /([a-z]+)/gi);
my %unique = map { $_ => 1 } @vars;
                                                                                
printf "Unique variables:  %s\n", join(',',keys %unique);