#!/usr/bin/perl -00
# bysub3 - sort by subject using hash records
use strict;
my @msgs = ();
while (<>) {
    push @msgs, {
        SUBJECT => /^Subject:\s*(?:Re:\s*)*(.*)/mi,
        NUMBER  => scalar @msgs,   # which msgno this is
        TEXT    => '',
    } if /^From/m;
    $msgs[-1]{TEXT} .= $_;
} 

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