I remember that STR2NUM has an interesting bug / feature that allowed it to return structure arrays (and function handles and more recently tables, etc.), but cell arrays and char arrays (and more recently strings) are explicitly checked for and are not returned. So your code must have relied on this (most likely) unintended, undefined, undocumented behavior:
str2num('struct(''x'',{''a'',pi})')
str2num('{''a'',pi}')
There seems to be no other way to return "an array containing both numbers and alphanumeric characters".
