If you have several variables which require this behavior, it is convenient to write a function (str_maker) which generates the closures for you.

use strict;
use warnings;
use feature 'say';
sub str_maker{
    my $fixed = $_[0];
    my $dynamic = \$_[1];
    my $closure = sub{
        return $fixed . ', ' . $$dynamic;
    };
    return $closure;
}
my ($aa, $bb);
my $static;

$static = 'Hello';
my $str1 = str_maker($static, $aa);
$static = 'Greetings';
my $str2 = str_maker($static, $bb);

$aa = 'Kevin';
say &$str1;     # Hello, Kevin

$bb = 'World';
say &$str2;     # Greetings World

$aa = 'Bill';
say &$str1;     # Hello, Bill

$bb = 'Aliens';
say &$str2;     # Greetings, Aliens
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Minor point: $a and $b are poor choices for variable names because they may clash with variables of the same name used in sort. I used $aa and $bb instead.