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in reply to Re^3: DB2 error: "Can't bind unknown parameter marker ':p4'"
in thread DB2 error: "Can't bind unknown parameter marker ':p4'"

Incrase DBI->trace level (3) and see what you get, compare that to "data studio tool" for similar

I guess the :p4 refers to  $sth->bind_param_inout(4,\$returnValue,20);, that is the 4th bind_param, but you only have three placeholders (?,?,?)

So unless mySchema.myStoredProcedure somehow has a placeholder (i've never used one), there is your problem