Heres another way to do it, which seems more legible:

use strict;
use warnings;

while (<DATA>){
    chomp;
    my @line = split /:/;
    print "$line[0]:$line[1]00:$line[2]:$line[3]:$line[4]00\n";
}

__DATA__
o:200806050852:b7199767:b24211012077030:200902232359
o:201103251457:b6235734:b2421100997663:201204022359
o:201403101021:b8015840:b24211013181843:201404072359
o:201407291225:b8759124:b24211001018862:201408262359
[download]

Output:

o:20080605085200:b7199767:b24211012077030:20090223235900
o:20110325145700:b6235734:b2421100997663:20120402235900
o:20140310102100:b8015840:b24211013181843:20140407235900
o:20140729122500:b8759124:b24211001018862:20140826235900
[download]