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

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

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

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

[download]

In reply to Re^4: is (?: ... ) capturing by ikegami
in thread is (?: ... ) capturing by jeanluca

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