If I begin a label with a number, PanX reports an error in the procedure.
Correct. Labels must begin with a letter. Numbers are allowed after the first character, however.
The same is true of variables and field names, though in that case you can surround with chevrons to force it to work. There is no such workaround with labels, they must begin with a letter.
Unfortunately, this restriction is not currently mentioned in the documentation. I have just submitted corrections for the 5 pages that mention labels - callwithin, callwithin(, farcallwithin, goto and shortcall.