#!/usr/bin/perl -00
# datesort - sort mbox by subject then date
use strict;
use Date::Manip;
my @msgs = ();
while (<>) {
    next unless /^From/m;
    my $date = '';
    if (/^Date:\s*(.*)/m) {
        ($date = $1) =~ s/\s+\(.*//;  # library hates (MST)
        $date = ParseDate($date);
    } 
    push @msgs, {
        SUBJECT => /^Subject:\s*(?:Re:\s*)*(.*)/mi,
        DATE    => $date,
        NUMBER  => scalar @msgs,
        TEXT    => '',
    }; 
} continue {
    $msgs[-1]{TEXT} .= $_;
}

for my $msg (sort {     
                        $a->{SUBJECT} cmp $b->{SUBJECT} 
                                       || 
                        $a->{DATE}    cmp $b->{DATE} 
                                       || 
                        $a->{NUMBER}  <=> $b->{NUMBER} 
                  } @msgs
         )
{
    print $msg->{TEXT};
}