my $r;
my $s = ...;

interface_control($s, \$r);

# use $r here
...

sub interface_control
{
    my ($s, $r_ref) = @_;
    my (@r, $v, @h);
    $s->get_handles(rtrs=>\@r, vars=>\$v, hosts=>\@h);
    $$r_ref = $r[0];    # dereference $r_ref and write to its referent
    ...