You actually don't need either one.

use constant SECOND  => 0;
use constant MINUTE  => 1;
use constant HOUR    => 2;
use constant DAY     => 3;
use constant MONTH   => 4;
use constant YEAR    => 5;
use constant WEEKDAY => 6;

my @time = localtime(time);

my $string =
    $time[WEEKDAY] + 1 . '-' .
    $time[MONTH] + 1   . '-' .
    $time[DAY]         . '_' .
    $time[HOUR]        . ':' .
    $time[MINUTE]      . ':' .
    $time[SECOND]
;
[download]

Or, if you're comfortable with join(), you could replace the last line with:

my $string =
    join('-', $time[WEEKDAY] + 1, $time[MONTH] + 1, $time[DAY])
  . '_' .
    join(':', @time[HOUR, MINUTE, SECOND])
;
[download]

But, it's probably better to work with any of the DateTime modules on CPAN. You might want to look at DateTime, which is the up-and-coming set of modules that should do for date/time manipulation/calculation that DBI did for database access.