Here's yet another way to do it. Don't be too distracted by the /x modifier which allows for whitespace in the RE. I just did that to hopefully make it a little easier to read.

All the 'work' is done inside the right side of the s/// operator. @array is there just to demonstrate that it works on every possible combination of foo.

use strict;
use warnings;

my @array = qw/foo Foo FOo FOO fOO fOo foO FoO/;
my $repl = 'bar';

print "@array\n";
 
s{
    \b(foo)\b
}{
    join '', map {
        substr( 
            substr( $1, $_, 1 ) ge 'a' ? lc $repl : uc $repl,
            $_, 1 
        )
    } 0 .. $+[0]
}eix foreach @array;

print "@array\n";
[download]

Enjoy!