Can you show a sample line from the log? If it is just anything followed by a space and the ip, this should be enough:

perl -i~ -pe ' s/(.*) ([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})
+$/$2 $1/;' 2001-01-02.log
[download]

I used the following data for testing:

234.234.234.234.asd.f2.35.346t.sdfg.aws.5332.6t.sdf.as4.23.4234 12.12.
+153.43
[download]

BTW, this is a shorter version of the regex: s/(.*) ((?:[0-9]{1,3}\.){3}[0-9]{1,3})$/$2 $1/