(Thoroughly untested.)

#!/usr/bin/perl

use strict;
use warnings;
# use diagnostics;

use File::Find;
use Date::Parse;

find \&process, 'tmp';
exit 0;

{
  my $output_dir;
  BEGIN { $output_dir = '/home/hynek/old-blog/new'; }

  sub process {
    if ( -f $_ && /\.html$/ ) {
      my ( $time, $text ) = parse( $File::Find::name );
      ( my $output_file = "$output_dir/$_" ) =~ s/html$/txt/;
      print_out( $text, $output_file );
      utime $time, $time, $output_file;
    }
  }
}

sub parse {
  my $input_file = shift;
  open my $in, "<:utf8", $input_file or die "Can't read $input_file: $
+!";
  my ( $time, $subject, $text );
  while ( <$in> ) {
    $time = Date::Parse::str2time( $1 ) and next
      if !$time && /\w+, (\w+ \d\d, \d\d\d\d)/;
    chomp;                 # is this *really* necessary for all lines?
    my $match_no;
    if ( $match_no = ( m%<h3 class="post-title">% ... m%</h3>% ) ) {
      next if $match_no == 1;
      $subject = $_       and next if /\w/;
      $text    = $subject and next if $match_no =~ y/E//;
    }
    if ( $match_no = ( m%<div class="post-body">% ... m%</div>% ) ) {
      next if $match_no == 1;
      last if $match_no =~ y/E//;
      $text .= "$_\n";          # why chomp earlier?  are you getting
                                # rid of DOS eol sequences?
    }
  }
  close $in or die "Failed to close $input_file: $!";
  $text =~ s,(\r|</?p>|^\s*),,gm;
  return ( $time, $text );
}

sub print_out {
  my ( $text, $filename ) = @_;
  open my $out, ">:encoding(iso-8859-15)", $filename or
    die "Cannot write to $filename: $!";
  print $out $text;
  close $out;
}
__END__
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