$name =~s/_/\s+/;
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Well, that's a bit silly, isn't it? You should understand that "\s+" in the left (regex-match) side of the s/// operator is supposed to mean "match one or more of any kind of white space character". And the right (replacement string) side of the s/// operator is supposed to say exactly what should replace the match.

So the reason why "\s+" can't do what you meant as part of the replacement string is: it doesn't say exactly what sort of white space character(s) -- or how many -- should replace the matched string. The same concept applies to any character-class, quantifier or grouping symbol used in a regex match: there's no way they can have the same meaning as replacement strings -- rather than being meaningless, they are simply taken "literally".