Note that in this last line &ob is used, while in the definition of the sub x it is plain ob that is used in ob-// returning -1 and not 2 for some unclear reason ...

In the definition of sub x the subroutine ob is not yet declared so the bareword ob is interpreted as an unquoted string. Then 'ob'-1 interprets 'ob' as 0 since it's not a number.