my $text = <<"";
adfRadfaUYBGsQWERaeYETEWoyMSn
nbPOIVcRCVVfOOPQbHbnRIIqWweRT

for my $match ( $text =~ /(?<=[A-Z]{4})([a-z])(?=[A-Z]{4})/g )
{
    print $match, $/;
}