Ah, I see. Well, I've already provided the building blocks, but they are well hidden. Let me expose them.

You need something along the lines of /[^$chars]*/, but instead of negatively matching chars, you want to negatively match a regexp.

The direct equivalent of
/[^$chars]*/
for regexps is
/(?:(?!$re).)*/

In context,

# Input the string.
my $in = do { local $/; <DATA> };

# Must move "pos" on a match.
# Zero-width match won't work.
my $start_pat = qr/^\S+/m;

# Break the input into paragraghs.
my @paras = $in =~ /
   \G
   (
      $start_pat
      (?: (?!$start_pat). )*
   )
/xgs;

# Manipulate the paragraghs.
@paras = map { "<$_>" } @paras;

# Recombine the paragraphs.
my $out = join '', @paras;

# Output the string.
print($out);

__DATA__
abc:
  asdf1
  asdf2

def:
  asdf3

ghi:
  asdf4
  asdf5
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