in reply to Re^3: is (?: ... ) capturing
in thread is (?: ... ) capturing

s/(\s+)\w/${1}_/g

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simplifies to 
s/(\s)\w/${1}_/g

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Now you can use a zero-width lookbehind to get the speed boost of avoiding captures. 
s/(?<=\s)\w/_/g

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Or in 5.10+: 
s/\s\K\w/_/g

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Re^5: is (?: ... ) capturing
by ysth (Canon) on Sep 07, 2009 at 17:18 UTC
[reply]

In Section Seekers of Perl Wisdom