# assume @a is your array
my $pos;
for my $i (0..$#a) {
    if ($a[$i] eq "Element_15") { $pos = $i; last; }
}
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$pos is either undef or the position of the first occurrence of "Element_15".

Hashes don't have any defined order to them, so the question doesn't make any sense if you are using a hash. If your "table" is an HTML table, that's a completely different situation.

If you happened to mean a database table then you first have to define the order, then get the value of the order column(s) for that row and then select one row that would be before that row in the ordering.

Assuming the table is named People, you want it sorted on LastName and then FirstName and want the person just before the person with ID 15 it could be something like this:

SELECT TOP 1 Previous.*
  from People as Previous
  join People as This on Previous.LastName < This.LastName or Previous
+.LastName = This.LastName and Previous.FirstName < This.FirstName
 WHERE This.ID = 15
 ORDER BY Previous.LastName DESC, Previous.FirstName DESC
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SELECT TOP 1 *
  from People
 WHERE LastName < 'Doe' or LastName = 'Doe' and FirstName < 'Joe'
 ORDER BY Previous.LastName DESC, Previous.FirstName DESC
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How (Not) To Ask A Question

This recipe for implementing sparse arrays using hashes might help with what you want.

On the other hand, if the Element_<number>s are all consecutive, you might be better off ditching hashes altogether and using a plain array. The previous-element problem becomes trivial, though searching an array for a known value is less simple.