$ perl -le'
my $Original = "AABBCCDDEEFFGGHH";
my $Modified = "bCCDdEEFfgGH";
print $Modified;
( $Modified, my $temp ) = ( $Original, $Modified );
$Modified =~ s/(.*?)(?i:$temp)(.*)/\L$1\E$temp\L$2\E/;
print $Modified;
'
bCCDdEEFfgGH
aabbCCDdEEFfgGHh
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