This really isn't a Perl question, it's a SQL question.

SQL is a declarative language, which means you describe what you want, rather than how to get it. You've stated you want to compare things in the same table, so you'll need aliases.

from table a join table b on -- the primary key
where a.idNumber = b.idNumber
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Next, you say the price difference is what you want, so:

select abs(a.price - b.price) as difference
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And finally, you want the two most recent items, which should be something like:

where a.date = (select max(Date) from table t1
      where t1.idNumber = a.idNumber)
and
     b.Date = (select max(Date) from table t2
     where t2.idNumber = b.idNumber)
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And if you put it all together, you get something like this:

select
      a.idNumber              -- Presuming you want
    , b.idNumber              --    these too.
    , abs(a.Price - b.Price)  -- the price difference
from
    table a join table b on SOMETHING -- the primary key
where
        a.idNumber = b.idNumber
    and a.date = (select max(Date) from table t1
         where t1.idNumber = a.idNumber)
    and b.Date = (select max(Date) from table t2
         where t2.idNumber = b.idNumber)
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That may or may not work, depending on your data model. But it should get you pointed in the right direction.