bash-2.05b$ cat xml2sgml.pl
#!/usr/bin/perl -w
use strict;
my $doctypePat = quotemeta ( '<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"' );
my $fl;
 
$fl = <>;
 
print $fl if $fl !~ /^\<\?xml/;
 
while(<>) {
        if (/$doctypePat/) {
                print <<EOT;
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN" [
<!ENTITY nwalsh "Norman Walsh">
]>
EOT
                <>;
                next;
        }
        s/article/book/g;
        if ( /\<\/bookinfo>/ ) { print $_; print "<article>\n"; next; }
        if ( /\<\/book>/ ) {  print "</article>\n"; print $_; next; }
        print $_;
}