why is it that changing it to "....0", "..0", ".0" and "" give the same output?

s///.0     ->    s/// . 0
s///..0    ->    s/// .. 0
s///...0   ->    s/// ... 0
s///...0   ->    s/// ... .0   ->    s/// ... 0
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The last two are identical, save for the use of different literals for zero.

The .. and ... operators are related, and behave the same given their arguments.

The . operator (concatenation) does not behave like the range operator.

>type file
123
abc
def

>perl -ne"print if s/.*?(?=abc)//.0" file
abc

>perl -ne"print if s/.*?(?=abc)//..0" file
abc
def
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