Using Date::Manip:

use strict;
use Date::Manip;

my $from = 'Wed Dec 29 08:17:04';
my $to   = UnixDate($from, "%y-%m-%d_%H:%M:%S");

print "$to\n";
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use POSIX qw/strftime/;
print strftime("%y-%m-%d_%H:%M:%S", localtime);
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#!/usr/bin/perl -wl

use strict;
use Date::Parse;

my $from = 'Wed Dec 29 08:17:04';

print str2time($from);

__END__
1104329824
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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]
;
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my $string =
    join('-', $time[WEEKDAY] + 1, $time[MONTH] + 1, $time[DAY])
  . '_' .
    join(':', @time[HOUR, MINUTE, SECOND])
;
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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.

my @d = ( localtime )[5,4,3,2,1,0];
$d[0]-=100;   # fix the year
printf("%02.d-%02.d-%02.d_%02.d:%02.d:%02.d", @d);
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04-11-29_11:29:37
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I want to convert a localtime date like so... I would appreciate any suggestions as to the most efficient and readable way to do this.

If by "efficient" you mean that you have a large number of these to translate, and that you really want to roll your own instead of using one of the several decent (but sometimes bloated) data manipulation packages, consider doing a hash lookup. Pre-generate a mapping like

  my %date_convert = (
    ...
    'Dec 29' => '04-12-29',
    ...
  );
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  my $day_re = '(?:Mon|Tue|...|Sun)';
  my $mon_re = '(?:Jan|Feb|...|Dec)';
  my $time_re = '\d\d:\d\d:\d\d';

  $text =~ s/$day_re ($mon_re \d+) ($time_re)/$date_convert{$1} . "_" 
+. $2/ge;
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You didn't show us where the year comes in, so that's an open issue you'd need to deal with.

Pregenerating the conversion hash is left as an exercise.

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";
    }
}
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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;
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