If you split on the paragraph separators (two or more consecutive linefeeds), and use capturing parens in the split, it's pretty easy:

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

def:
  asdf3

ghi:
  asdf4
  asdf5
EOF

my @tkns = split /(\n{2,})/;
my @pars;
for ( @tkns ) {
    if ( /^\n+$/ ) {
        $pars[$#pars] .= $_;
    } else {
        push @pars, $_;
    }
}

printf "found %d paragraphs:\n", scalar @pars;
print "<", join( "><", @pars ), ">\n";
[download]

That prints:

found 3 paragraphs:
<abc:
  asdf1
  asdf2

><def:
  asdf3

><ghi:
  asdf4
  asdf5
>
[download]