You can check that with the ref function:

use strict;
my $var = [qw(this is an array)];
print ref $var;

my $var = "This is a scalar";
print ref $var;

my $var = { message => "This is a scalar" };
print ref $var;
[download]

But you really, really want to use the ForceArray configuration parameter to make all your results arrays, whether there is one or more elements in the XML.