Not quite \w?+ (which could be \w+ just as easily), as that gives you digits as well, which it sounds like you don't want.  Instead, use [a-z]+, which matches against variable names (alpha only, in this case).  The /gi lets you match all occurrences (g = global) without regard for case (i = ignore case).

If you have to match alphanumerics (eg. a7), then use something like ([a-z]\w*0) instead.

Of course, any problem requiring unique results just shouts out for a hash.

You could try something like this:

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);
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