Here is Another Way:

my $foo = "AbC";
my $bar = "Ducks";
$string = "ABCDEFGHIJabc";

$string =~ s/($foo)/
for (my $i=0, my $j=length $1, my $k=length $bar;
$A=substr($foo,$i,1) and $B=substr($bar,$i,1);
$i++) {
  if (
  $A ge 'A' and $A le 'Z') {
  substr($bar,$i,1) = uc($B);
}
}$bar
/giex;
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