A metacharacter is a character, or a sequence of characters, that has a special meaning in a computing application. It might be used to represent another set of characters, an unprintable character, or a logical operation. For example, in some applications, \n is interpreted as a newline. (In this case, a backslash is used to escape the n character, indicating that it should be interpreted as a metacharacter.)
$data =~ s/\n//;
This statement says "assign a new value to $data which substitutes any newline in $data with nothing."
For a list of common metacharacters, and examples of how to use them, please see our Regular Expressions Quick Reference.