FYI, you can test ENV variables from the command line like so:

$ HTTP_USER_AGENT=MSIE perl myscript.cgi
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Or:

$ HTTP_USER_AGENT=FireFox perl myscript.cgi
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That is, put the environment variable name plus what value you want it to have, separated by an equals sign, immediately preceding your invocation of the script. Just a handy way to test things like that out.

But I'll second Joost's advice (though not to the letter) and suggest dealing with goofy IE CSS implementations through plain CSS.

s**lil*;  $*=join'',sort split q**;  s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)
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