An approach using split (although conceptually similar to tye's) and splitting on groups of three lines because fourteen lines necessitates a very tedious example.
>perl -wMstrict -le
"my $s =
qq{foo \n bar \n baz \n fee \n fie \n foe \n aa \n bb \n cc \n};
print qq{[[$s]]};
;;
my @fields = split m{ (?: [^\n]* \n){3} \K }xms, $s;
print qq{[[$_]]} for @fields;
"
[[foo
bar
baz
fee
fie
foe
aa
bb
cc
]]
[[foo
bar
baz
]]
[[ fee
fie
foe
]]
[[ aa
bb
cc
]]
Update: Also tested/works in cases in which: last 'line' does not end in newline; exactly 3 lines processed; fewer than 3 lines processed; lines consist only of newlines; etc. – I think, in fact, all possible cases.
