So - so far all of the other responses have assumed you can read a line at a time. Sometimes you can't. Well - here is one possible option that will let you do what you want (assuming I know what you want) on a single string.
my $str = "<html>
<title>Hi</title>
<body>
Some other lines
of stuff
</body>
</html>
";

$str =~ s{ ^(.+)$
           (??{
             lc(substr $^N, 0, 7) eq "<title>"
               ? '(?!)'
               : '(?=)'
           })
         }{<para>$1</para>}xgm;

print $str;

# prints

<para><html></para>
<title>Hi</title>
<para><body></para>
<para>Some other lines</para>
<para>of stuff</para>
<para></body></para>
<para></html></para>

[download]

This uses the delayed regex feature that lets you put a regex in a later point. It takes whatever is in $^N (the last match) and sees if begins with <title>. If it begins with title we put in '(?!)' which is a negative look ahead that will always fail - otherwise we put in '(?=)' which is a positive look ahead that will always succeed.

Unfortunately we have to do the testing to see if each line begins with title with with string methods rather than a nested regex. If we used a regex inside the (??{}) we would confuse the regex engine and in many (if not all) cases cause a segfault. But string methods often work just fine.

my @a=qw(random brilliant braindead); print $a[rand(@a)];

In reply to Re: Negative look ahead by Rhandom
in thread Negative look ahead by Anonymous Monk

Title:
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