The following will do your conversion for you, assuming that you're trying to do something more complex than just output a localtime value in a different format:

use strict;
use warnings;

my $date = 'Wed Dec 29 08:17:04';
$date = convert($date);
print $date;

BEGIN {
    my %c_mon = ('Jan',1,'Feb',2,'Mar',3,'Apr',4,'May',5,'Jun',6,
               'Jul',7,'Aug',8,'Sep',9,'Oct',10,'Nov',11,'Dec',12);
    sub convert {
        my ($mon, $day, $rest) = $_[0] =~ /\w+ (\w+) (\d+) (.+)/; $mon
+ = $c_mon{$mon};
        @_ = localtime(); $_[4]++; my $year = $_[5] % 100;
        $year-- if "$mon$day" gt "$_[4]$_[3]"; $year = 99 if $year == 
+-1;
        return sprintf('%02d', $year) . "-$mon-$day\_$rest";
    }
}
[download]

As you can see, much of the effort is expended in determining the year, which isn't given in the original date. I've assumed that a date with a month / day after the current month / day is one in the previous year, and that all other dates are in the current year.

If you just want a localtime value in the format given, the following works:

use strict;
use warnings;

@_ = localtime();
my $date = sprintf('%02d-%02d-%02d_%02d:%02d:%02d',
                    $_[5] % 100, $_[4] + 1, $_[3], $_[2], $_[1], $_[0]
+);
print $date;
[download]

I'm assuming you want a 24-hour time.