Putting aside the very odd goal...

The following regex will work for you.

By using a look behind assertion \K and a look ahead assertion, we can simply inject " selected" into the appropriate option: 
$selection =~ s/<option [^>]*\K(?=>\s*\Q$value\E\b)/ selected/;

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In reply to Re: regex question: How do I modify in place in only one character class? by wind
in thread regex question: How do I modify in place in only one character class? by aturtle

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