I'm not entirely sure what you're trying to accomplish, but I'll give it a shot. :)

my $row = $dbh->selectall_arrayref(...)
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$row will be a reference to an array of arrayrefs. E.g. ...

$row->[0]->[0]
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... is the first column of the first row of output. If you want an array of hashrefs, use selectall_hashref():

my $row = $dhb->selectall_hashref(...)
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$row->[0]->{'g_afsluttet'} is the column named 'g_afsluttet' in the first row of output. Now, if I understand your question right, you want to modify one of the columns of your output. You want to replace the value of the column named 'g_afsluttet' with con_date(<value>) for every row in the output? If that's the case, you can do it like this:

my $rows = $dbh->selectall_hashref(...);
...
foreach my $hashref (@$rows)
{
    $hashref->{'g_afsluttet'}
        = con_date($hashref->{'g_afsluttet'});
}
...
$tmpl->param(LOOP => $rows);
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-Matt