for ( my $c= 0; $c<= $#clk_output; $c++) {
   for ( my $d = 0; $d <= $#{$clk_output[$c]}; $d++ ) {
        $clk_new = join '_', @{$clk_output[$c]};
        push @clk_new, $clk_new;
    }
}
print Dumper \@clk_new;