s/((\n) ([^0-9])+ (-)* (Aa-Zz)*) | ((\n) (\d{3}) (-)* (Aa-Zz)*)/$2$3/gx;
# 12    3         4    5           67    8       9    0

##</code><code>##

use strict;
use warnings;

my $wholeBallOfWax = do {local $/; <DATA>};
my @records = split /(?<=\n)(?=\d+-)/, $wholeBallOfWax;

s/\n+$/\n/s for @records;

print join "---\n", @records;

__DATA__
1-12
last non-blank field


2-10
data
more data

3-21
stuff
more stuff
Lots of stuff
so much stuff there is no following empty field
4-73

Sneeky record with a blank field in the middle!

5-00
Last record

##</code><code>##

1-12
last non-blank field
---
2-10
data
more data
---
3-21
stuff
more stuff
Lots of stuff
so much stuff there is no following empty field
---
4-73

Sneeky record with a blank field in the middle!
---
5-00
Last record