use strict;
my $str = '<div>
test
test
test
</div>
<pre>
Line one
Line two
Line three
Line four
Line five
Line six
</pre>';
my $regex = qr~(?<!<pre>)\R(?!</pre>)(?=((?!<pre>).)*</pre>)~sp;
my $subst = '<br />';
my $result = $str =~ s~$regex~$subst~rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html