"the lazy  brown   fox    jumped..."
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This should work:

$foo =~ s/(?<= )( +)/'&nbsp;' x length($1)/ge
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But I would like to expand the question a bit. I would like to see all spaces removed with the exception of just one separating the words, and then have that replace with the non-breaking space. So:

"the lazy brown fox jumped..."
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Can be solved thusly:

$foo =~ s/ {2,}/ /g;
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-enlil