join($column[2],$column[4])
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Is probably not doing what you think it is doing.

join EXPR,LIST

    Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string

In your case, LIST is a single item, so the result of the join will be just $column[4].

You probably wanted:

$motherID = "$column[2]$column[4]" # Interpolation
# OR (They both do the same thing)
$motherID = $column[2] . $column[4]; # Concatenation
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