Take a look at perlrequick. It gives you a quick overview of Regular Expressions in Perl. My explanation of your code is as follows:

1. The m tells perl to start a match.
2. The | character is used as a delimiter. Using / is normal but any delimiter can be used. If you use / then you don't need to start with an m
3. The $input is interpolated (The content of the variable is placed in the regular expression)
   
4. The o modifier signifies that the regular expression is compiled once. Changing the value in $input will not change your regex.
5. The whole regular expression is matched against the value held in $_ by default. You would use the =~ operator to match a particular variable.