Thanks. I did it slightly modified borrowing the -l from kennethk above:

(Windows 7 x64 / Strawberry Perl v5.18.1 MSWin32-x64-multi-thread)

VinsWorldcomC:\Users\VinsWorldcom\tmp> perl -alnF"/\./" -e "print join
+ q{.}, reverse @F" zones.txt
10.100.1
10.100.2
10.100.3
[download]

Nice. And golf it down with -E/say:

perl -anlF"/\./" -E"say join q{.},reverse@F" zones.txt
[download]

Golf? -p and $_= save you one space over -n and say:

perl -aplF"/\./" -E"$_=join q{.},reverse@F" zones.txt
[download]

and a manual split takes less than -F (assuming you don't get flags for free):

perl -lpE"$_=join q{.},reverse split/\./" zones.txt
[download]

Using $, in place of join saves you 2, except then you lose $_=

perl -lnE"$,=q{.};say reverse split/\./" zones.txt
[download]

but you still win w/ a regex:

perl -pE"s/(.+)\.(.+)\.(.+)/$3.$2.$1/" zones.txt
[download]

and to me, that one seems a whole lot more obvious than the other offerings.

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#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.