#!/usr/bin/perl

        # bysub1 - simple sort by subject
        my(@msgs, @sub);
        my $msgno = -1;
        $/ = '';                    # paragraph reads
        while (<>) {
            if (/^From/m) {
                /^Subject:\s*(?:Re:\s*)*(.*)/mi;
                $sub[++$msgno] = lc($1) || '';
            }
            $msgs[$msgno] .= $_;
        }
        for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
            print $msgs[$i];
        }

    Or more succinctly,

        #!/usr/bin/perl -n00
        # bysub2 - awkish sort-by-subject
        BEGIN { $msgno = -1 }
        $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
        $msg[$msgno] .= $_;
        END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }



# {%WRAPPER _|EQ|_ "_c_doc__w_plain.txt"%}
# {%PREVENT_LAYOUT_PROCESS _|EQ|_ "0"%}