Help for this page
#!/usr/bin/perl ... A line with_two_underscores. ~A line with an_underscore starting with a tilde. ~A line with_two_underscores starting with a tilde.
A line with an_underscore. A line with_two_underscores. ~A line with an+underscore starting with a tilde. ~A line with+two_underscores starting with a tilde.
s/( ^ ~ .*? ) _ /$1+/xg;