/(?=.*\.)/ and /[^.]*/ are not the same thing at all.

'booga'  =~ /[^.]*/            matches.
'booga.' =~ /[^.]*/            matches.

'booga'  =~ /(?=.*\.)/         does not match.
'booga.' =~ /(?=.*\.)/         matches.


'4.6.8'  =~ /[^.]*\.(\d)/      returns 6 in $1.
'4.6.8'  =~ /(?=.*\.)(\d)/     returns 4 in $1.
'4.6.8'  =~ /((?=.*\.))\1(\d)/ returns 8 in $2. .* matched a "4.6"
'4.6.8'  =~ /.*\.(\d)/         returns 8 in $1. Lookahead not needed.
'4.6.8'  =~ /(?:(?!\.).)*(\d)/ returns 6 in $1. You want neg lookahead
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/(?=.*\.)/ requires a "." in the string. /[^.]*/ does not requires a "." in the string. /[^.]*/ is equivalent of the negative lookahead /(?:(?!\.).)*/. The advantage of the negative lookahead version is that you can negatively match more than one character: /(?:(?!$re).)*/. If I only wanted to negatively match one character, I'd use [^...], even in Perl.

Side thought: Hum, It would be nice if /(?:(?!$re).)*/ could be shortcutted to /(?^$re)/, since that's the typical use of a negative lookahead.