You could use some fancy regex and use lookahead or lookbehind to make sure you are not inside a html tag.

Such as s/($seekword)(?!>)/<b>$1<\/b>/ig;

(I would also suggest either using an escape for </b> or use another delimitor for your s/// such ask s{($seekwork)(?!>)}{<b>$1</b>}ig;.

This code is of course untested and off the top of my head, I suggest looking up perlre, and reading up on the subject of (?!patern) in regexs.

updated: Or as noted above, try the HTML::Parser module. My trick is a quick and dirty aproach that might break in some cases.