You already have the interesting parts to change code:

 # my $date = $arr_ref->$datecol;

# $date =~ s/ //;
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I suggest that you take the database out of the problem and start out with a simple subroutine that converts the date to the format you want:

sub fix_oracle_date {
    my( $date ) = @_;

    # $date =~ s/ //;

    return $date
}
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... and then test your subroutine using (for example) Test::More, which makes this very easy:

use Test::More;

is fix_oracle_date('08/02/2018'), '8/2/2018', "Correct date for 8/2/20
+18";
is fix_oracle_date('08/03/2018'), '8/3/2018', "Correct date for 8/3/20
+18";
is fix_oracle_date('08/04/2018'), '8/4/2018', "Correct date for 8/4/20
+18";
is fix_oracle_date('08/13/2018'), '8/13/2018', "Correct date for 8/13/
+2018";

done_testing;
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Now you have a good testbed and you can work on implementing fix_oracle_date until it returns the correct data for your input.

Maybe you want to start with the implementation I gave you in my previous reply?

See also How to ask better questions using Test::More and sample data, which outlines this process.