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

In reply to Re: Search and replace by Enlil
in thread Search and replace by Anonymous Monk

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