...
       if ( not defined $most_recent{$col1}{$col2} or $most_recent{$col1}{$col2}->{dtime} lt $dtime )
        {
                $most_recent{$col1}{$col2}->{dtime} = $dtime;
                $most_recent{$col1}{$col2}->{col3}  = $col3;
                $most_recent{$col1}{$col2}->{col4}  = $col4;
        }
}
print "Most recent:\n";
foreach my $col1  ( keys %most_recent ) {
        foreach my $col2 ( keys %{$most_recent{$col1}} ) {
                print "$col1, $col2, $most_recent{$col1}{$col2}->{dtime}, $most_recent{$col1}{$col2}->{col3}, $most_recent{$col1}{$col2}->{col4}\n";
        }
}