You could use a combination of lookbehind and lookahead: 

perl -pe "s[(?<=\d{5})\s+(?=\d{8})][\n]g" t.txt

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re^4: repeat data at one line and grep as pre-set format by BrowserUk
in thread repeat data at one line and grep as pre-set format by benlaw

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