well, you'd probably want to read in the log file as an array:

open(ERRLOG,"myerror.txt");
@file = <ERRLOG>;
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then loop through the resultant array, using something like Date::Parse to convert the dates to unix time (ex: "Thu Jan 26 10:04:56 2006" becomes 1138291496), and splice off any elements older than the given time:

foreach $line (0..$#file)
{
  # grab the date and dump into $date
  $date = $1 if $file[line] =~ m/^\[(.*?)\]/;

  # parse it to unix time
  $date = str2time($date); # method from Date::Parse

  # if its older than a month (30 days specifically)
  if($date < (time-2592000)) # 2592000 secs = 30 days
  { splice @file,$line,1; }
}
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then just output the array (whats left of it, anyway) back to the file. Note that this code hasnt been tested, by the way.