if ($string =~ m!^(\w{4})$!) {
	$string =~ tr/[a-z]/[A-Z]/;
	# or $string = uc $string;
	
	# do something
	} else  {
	# do some error
}

##</code><code>##

if ($string =~ s!^(\w{4})$!uc($1)!e) {
	
	# do something
	} else {
	
	# do some error
	# print "Location: $error_url"
}