Try like this,

use strict;
use warnings;
my $str = <<EOF;
abc:
  asdf1
  asdf2

def:
  asdf3

ghi:
  asdf4
  asdf5
EOF

my @str = split/(?=\n+\w)/, $str;

print "$_" for @str;

#or

$str =~ s/(^|\n+)(\w)/$1<\/p>\n<p>$2/gsi;
$str =~ s/^(<\/p>)\n*(.+)$/$2$1/gsi;

print $str;
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