my %hash = (
    'foo' => 'bar',
    'Foo' => 'Bar',
    'FOO' => 'BAR',
    );

my $string = "foo Foo foo FOO";

print "old: $string\n";
foreach (keys %hash)
{
    $string =~ s/($_)/$hash{$1}/g;
}

print "new: $string\n";