#!perl

use v5.14;
use strict;
use warnings;

use DateTime;
use DateTime::Format::Strptime;

my $parser = DateTime::Format::Strptime->new(
    pattern => '%Y%m%d %H:%M:%S'
);

my $format = '%m/%d/%Y %r';

while (my $timestamp = <DATA>) {
    my $dt = $parser->parse_datetime($timestamp);

    $timestamp = $dt->strftime($format);

    # Remove leading zeros from months, days and hours,
    # but not from minutes or seconds...
    $timestamp =~ s{(?<!:)0(?=[1-9][/:])}{}g;

    say $timestamp;
}

exit 0;

__DATA__
20040805 18:31:00
20050106 10:54:27
20050302 01:23:35
20100808 20:00:16
20110501 18:09:44
20110909 20:02:42
20130408 18:09:03