Putting dot-star next to an anchor is pointless. Just throw out the anchor and the dot-star. That leaves you with:

/\/\*((?:(?<!throw))/
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You've got capturing parentheses around non-capturing parentheses, around a negative lookbehind. You only need the parens for the negative lookbehind:

/\/\*(?<!throw)/
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Ok, now you've matched "/*", and at that point, you're looking back to ensure that what comes before you isn't "throw". It can't be, because it ends in "/*". You can't really check everything up to the "/*" with a negative lookbehind, because negative lookbehinds can't be variable-length, and your line can be. You can do it with negative lookahead:

if ($line =~ /^(?:(?!throw).)*?\/\*/)
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That will be any number of characters that isn't the start of "throw", followed by "/*". The *? makes it take the first "/*" rather than the last.

Please see this node about YAPE::Regex::Explain for a helpful module.