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.'

'___'
'___'

'~___'
'~+++'
[download]

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 my @test_set = (
   'degenerate and simple cases',
   [ '',    '',    'empty line'        ],
   [ ' ',   ' ',   'single space'      ],
   [ '~',   '~',   'single tilde'      ],
   [ '_',   '_',   'single underscore' ],
   [ '~_',  '~+',  'tilde, underscore' ],
   [ '_~',  '_~',  'underscore, tilde' ],
   [ '__',  '__',  'multiple underscores'        ],
   [ '~__', '~++', 'tilde, multiple underscores' ],
   [ '__~', '__~', 'multiple underscores, tilde' ],
   'more complicated cases',
   [ 'A line with an_underscore.',
     'A line with an_underscore.',
     'no leading tilde, text w/one underscore'
     ],
   [ 'A line with_two_underscores.',
     'A line with_two_underscores.',
     'no leading tilde, text w/two underscores'
     ],
   [ '~A line with an_underscore starting with a tilde.',
     '~A line with an+underscore starting with a tilde.',
     'leading tilde, text w/one underscore'
     ],
   [ '~A line with_two_underscores starting with a tilde.',
     '~A line with+two+underscores starting with a tilde.',
     'leading tilde, text w/two underscores'
     ],
   );
 ;;
 VECTOR:
 for my $ar_vector (@test_set) {
   if (not ref $ar_vector) {
     note $ar_vector;
     next VECTOR;
     }
   ;;
   my ($input, $expected, $comment) = @$ar_vector;
   ;;
   $input =~ s{ (?: \G (?! \A) | \A ~) .*? \K _ }{+}xmsg;
   is $input, $expected, $comment;
   }
 ;;
 done_testing;
 ;;
 exit;
"
# degenerate and simple cases
ok 1 - empty line
ok 2 - single space
ok 3 - single tilde
ok 4 - single underscore
ok 5 - tilde, underscore
ok 6 - underscore, tilde
ok 7 - multiple underscores
ok 8 - tilde, multiple underscores
ok 9 - multiple underscores, tilde
# more complicated cases
ok 10 - no leading tilde, text w/one underscore
ok 11 - no leading tilde, text w/two underscores
ok 12 - leading tilde, text w/one underscore
ok 13 - leading tilde, text w/two underscores
1..13
ok 14 - no warnings
1..14
[download]

Update 2: Removed an extraneous  VECTOR: label that had crept into SSCCE code example. Code function unchanged.