This should work with some modifications to better suit your purposes:

while ( <DATA> ) {
  print unless m/##(\d+)## \1/;
}


__DATA__
REG-NZL-PUB.1981-263 ##1981## 19810922
REG-NZL-PUB.1981-263 ##1981## 19820922
REG-NZL-PUB.1981-263 ##1981## 19830922
REG-NZL-PUB.1981-263 ##1981## 19840922
REG-NZL-PUB.1981-263 ##1982## 19810922
REG-NZL-PUB.1981-263 ##1983## 19810922
REG-NZL-PUB.1981-263 ##1984## 19810922
REG-NZL-PUB.1981-263 ##1985## 19850922
[download]

Resulting in:

REG-NZL-PUB.1981-263 ##1981## 19820922
REG-NZL-PUB.1981-263 ##1981## 19830922
REG-NZL-PUB.1981-263 ##1981## 19840922
REG-NZL-PUB.1981-263 ##1982## 19810922
REG-NZL-PUB.1981-263 ##1983## 19810922
REG-NZL-PUB.1981-263 ##1984## 19810922
[download]

Note that I am assuming that the patterns of the lines in question are all the same.

-enlil