This may give you an hint. It assumes current working directory. You can adapt it easilly.

#!/usr/bin/perl -w

use Date::Calc qw(Delta_Days);

my $dirname = ".";

my (@files, @log_lines);
my %months = ( 'Jan' => 1,
               'Feb' => 2,
               'Mar' => 3,
               'Apr' => 4,
               'May' => 5,
               'Jun' => 6,
               'Jul' => 7,
               'Aug' => 8,
               'Sep' => 9,
               'Oct' => 10,
               'Nov' => 11,
               'Dec' => 12,
             );

# get the list of text files
opendir DIR, "$dirname";                                      
@dircontent = grep { /\.txt$/ } readdir(DIR);
closedir DIR;

# Compute today's date
my @today = (localtime(time))[3..5];
$today[1]++;
$today[2] += 1900;

# foreach text file, compute lines condition
foreach $filename (@dircontent) {
  open FILE, "<$filename";
  while(<FILE>) {
    my $line = $_;
    # see if line matches date format
    if($line =~ /^\[(\w{3}?) (\d{2}?)[^\]]*(\d{4}?)(.*)$/) {
      # assumed 'one month older' as being over 30 days.
      # if the line is older than 30 days, save it into an array
      # and write it later. If not, write it to a temp file
      (Delta_Days($3, $months{$1}, $2, $today[2], $today[1], $today[0]
+) > 30 )
        ? push @log_lines, $line : keep_line($line);
    } else {
      keep_line($line);
    }
  }
  close FILE;
  # The following lines restore the initial file but without old lines
  unlink $filename;
  rename "temp.tmp", $filename;
}

# archive old lines
open ARCH, ">>archive.arc";
foreach $line (@log_lines) {
  print ARCH "$line";
}
close ARCH;

# Just append a line to temp file
sub keep_line {
  my $line = shift;
  open  TEMP, ">>temp.tmp";
  print TEMP "$line";
  close TEMP;
}
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