perl -lwe 'use strict; print(join("/",split(/(?!\.\d)/,"123.456.78.1")))'
The idea is that a split point is any point in the string which is preceeded by a dot followed by a digit.
In your code you have a negative look-ahead assertion (?!\.\d).
In the explanation below you talk about "a point in the string which is preceeded by". For something like that you need a positive look-behind assertion.
Maybe you just mixed them up? Using (?<=\.\d) works as expected for me.

In reply to Re: split on zero-length pattern by tinita
in thread split on zero-length pattern by rovf

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