rr_add is the name of a function, right? And you're putting a different function name in $var1? This isn't a simple case of interpolation, because it modifies the flow of your program.

That's not to say you can't do it, of course. You can either use eval, as in:

$var1=rr_add
$update->push(update => eval "${var1}('foo.example.com. 86400 A 192.16
+8.1.2'))");
[download]

or else you can use a symbolic reference:

no strict 'refs';
$var1=rr_add
$update->push(update => $var1->('foo.example.com. 86400 A 192.168.1.2'
+))");
[download]

Both of these code snippets are untested, but they should get you pointed in the right direction.