Another approach, also using List::Util::max(), but using an array slice rather than splice:

c:\@Work\Perl>perl -wMstrict -le
"use List::Util qw(max);
 ;;
 my @ra = qw(a b c d e f g h);
 ;;
 my $n = 3;
 for my $i (0 .. $#ra) {
   my @triplet = @ra[ max(0, $i - $n + 1) .. $i ];
   printf q{'%s'  }, join '', @triplet;
   }
"
'a'  'ab'  'abc'  'bcd'  'cde'  'def'  'efg'  'fgh'
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(The call to  max() could be replaced with a  ?: ternary if necessary.)