DB<125> @combis = glob("{a,a1}{b,b1}{c,c1}")
 => ("abc", "abc1", "ab1c", "ab1c1", "a1bc", "a1bc1", "a1b1c", "a1b1c1
+")
[download]

glob takes a string which you can construct by replacing every "a" with "{a,A}" and so on...

see also Re: Substitution of a particular letter in all combinations of positions in word (see inside)

UPDATE

  DB<129> $s = "aabc"
 => "aabc"

  DB<130> %r = ( a=>A, b=>B, c=>C )
 => ("a", "A", "b", "B", "c", "C")

  DB<131> $s =~ s/(\w)/{$1,$r{$1}}/g
 => 4

  DB<132> $s
 => "{a,A}{a,A}{b,B}{c,C}"

  DB<133> print "$_\n" while glob($s)
aabc
aabC
aaBc
aaBC
aAbc
aAbC
aABc
aABC
Aabc
AabC
AaBc
AaBC
AAbc
AAbC
AABc
AABC
[download]