>perl -wMstrict -le
"use List::MoreUtils qw(part);
 ;;
 my @strings = qw(X XY  AAA  WXYZ VWXYZ  BBBCCC  TUVWXYZ);
 ;;
 use constant {
   X3  => 0,
   NX3 => 1,
   };
 my @indices =
   part { length($strings[$_]) % 3 ? NX3 : X3 } 0 .. $#strings;
 ;;
 print qq{indices of strings whose lengths are...};
 print qq{ even multiples of 3: @{$indices[X3]}};
 print qq{ non-even multiples of 3: @{$indices[NX3]}};
 ;;
 my $aminos = join '', @strings[ @{$indices[X3]} ];
 print qq{amino acids: '$aminos'};
"
indices of strings whose lengths are...
 even multiples of 3: 2 5
 non-even multiples of 3: 0 1 3 4 6
amino acids: 'AAABBBCCC'