a simple solution demonstrated under the Perl debugger:

  DB<1> @a = a..k;

  DB<2> x @a
0  'a'
1  'b'
2  'c'
3  'd'
4  'e'
5  'f'
6  'g'
7  'h'
8  'i'
9  'j'
10  'k'
  DB<3> for my $count (0..$#a)  { $start = $count - 2; $start = 0 if $
+start <0; print "@a[$start..$count]\n";}
a
a b
a b c
b c d
c d e
d e f
e f g
f g h
g h i
h i j
i j k
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