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use warnings;
use strict;
...

{ phone => { callTo => ["X"] } }
{ phone => { callTo => ["Y", "Z"] } }

use XML::Simple qw/:strict XMLin/;

...
{ phone => { callTo => "X" } }
{ phone => { callTo => ["Y", "Z"] } }
{ phone => { callTo => ["X"] } }

foo( "one" );
foo( ["two", "three"] );
...
<one>
<two>
<three>