c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my @lines = (
   '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.',
   );
 ;;
 for my $line (@lines, @ARGV) {
   print qq{'$line'};
   $line =~ s{ (?: \G (?! \A) | \A ~) .*? \K _ }{+}xmsg;
   print qq{'$line' \n};
   }
"  "___"  "~___"
'A line with an_underscore.'
'A line with an_underscore.'

'A line with_two_underscores.'
'A line with_two_underscores.'

'~A line with an_underscore starting with a tilde.'
'~A line with an+underscore starting with a tilde.'

'~A line with_two_underscores starting with a tilde.'
'~A line with+two+underscores starting with a tilde.'

'___'
'___'

'~___'
'~+++'