If I enter value '000123' value into Bind variables dialog box for :p_num bind variables I get '123' value inserted into a table
I think you mean entering ONLY the digits between the quotes and NOT entering the quotes themselves don't you?
What is the full version of Sql Dev that you are using? That was a bug in previous versions since if you actually enter what you posted (i.e. including the quotes) then the enclosing quotes are included in the data value which leaves no way to actually enter the digits only.
I am using Version 184.108.40.206 Build 15.21
And you are right, I enter only digits without quotes