use strict;
use warnings;

my $str;

while (<DATA>){	
	chomp;
	($_ =~ m/^\w/) ? ($str .= "<\/p>\n<p>$_\n") :($str.="$_\n");	
}
$str =~ s/^(<\/p>)(.+)$/$2$1/gsi;
print $str;

__DATA__
abc:
  asdf1
  asdf2

def:
  asdf3

ghi:
  asdf4
  asdf5