It all depends on how you are implementing your "table." If it is an array, you could do something like this:

# 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.