$input='Cat cat';

$input =~ s#\b([Cc]at)\b#<span class="topic">$1</a>#g;

print $input;
#output
#<span class="topic">Cat</a> <span class="topic">cat</a>
[download]

PerlMonks lovingly hand-crafted by Tim Vroom.
PerlMonks went on a couple dates, and then decided to shack up with The Perl Foundation.
Marvelous Managed Hosting and Bandwidth Generously Provided by pair Networks
Built with the Perl programming language.