Well, people should read documentation. If you see "qr" and don't understand what it does, the first thing you should do is read the docs -- don't just assume you know how it works.

From perldoc -f qr

       qr/STRING/

       qx/STRING/

       qw/STRING/
               Generalized quotes.  See the Regexp Quote-Like
               Operators entry in the perlop manpage.

From perldoc perlop

       qr/STRING/imosx
               This operators quotes--and compiles--its STRING as
               a regular expression.  STRING is interpolated the
               same way as PATTERN in `m/PATTERN/'.  If "'" is
               used as the delimiter, no interpolation is done.
               Returns a Perl value which may be used instead of
               the corresponding `/STRING/imosx' expression.