#!/usr/bin/perl -w
use strict;

my $str= "aa<bb>aa<U>aa</bb>aa</U>aa";
$str =~ s#<bb>(.*?)</bb># my $x= $1; $x =~ s/a/bb/g; $x #ge;
$str =~ s#<U>(.*?)</U>#\U$1#g;
print "($str)\n"
__END__
prints (aabbbbBBBBAAaa)