Use split to split a string into a list.

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

use List::Util qw{ sum };

my ($x, $y) = (2, 3);

my $opt = "$x, $y";
my @opts = split /,\s*/, $opt;
print sum(@opts);
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The dashes are just a convention. They were somehow common at the time Perl/Tk was created, so there might be other modules using them. See the explanation of "unary -" in perlop.