my %Tone =( A => 0, B => 2, C => 3, D => 5, E => 7, F => 8, G=>10);
my @Notes;
foreach my $natural ('A'..'G') {
  my $i = $Tone{$natural};
  my $iFlat = $i ? $i-1 : 11;

  $Tone{$natural.'#'} = $i+1;
  $Tone{$natural.'b'} = $iFlat;

  $Notes[$iFlat][0]=$natural.'b';
  $Notes[$i][0] = $natural;
  $Notes[$i][1] = $natural;
  $Notes[$i+1][1]=$natural.'#';
}

#print "Tone: ". Dumper(\%Tone);
#print "Notes: ". Dumper(\@Notes);


sub transpose {
  my ($key, $aChord, $toKey) = @_;
  my @aTransposed;
  my $iShift = $Tone{$toKey} - $Tone{$key};
  my $iIndex = ($toKey =~ /^F|.b$/) ? 0 : 1; # F uses flats

  push @aTransposed, $Notes[($Tone{$_}+$iShift) % 12][$iIndex]
    for @$aChord;
  return \@aTransposed;
}

# demo
local $"='-';
for ('A'..'G','Eb','Bb') {
  print "C-E-G => $_ : @{transpose('C',[qw(C E G)],$_)}\n";
}

# which outputs 
C-E-G => A : A-C#-E
C-E-G => B : B-D#-F#
C-E-G => C : C-E-G
C-E-G => D : D-F#-A
C-E-G => E : E-G#-B
C-E-G => F : F-A-C
C-E-G => G : G-B-D
C-E-G => Eb : Eb-G-Bb
C-E-G => Bb : Bb-D-F