Read perldoc perlop (or perlop), specifically the section about qq. It is the "double quote" operator, that is, it allows you to specify the delimiters for a double-quoted string.

Often, it also helps to let Perl tell you how it sees some code:

perl -MO=Deparse -e " $key=substr ($input,0, index($input,qq/ /,-1));"
[download]

gives me

C:\>perl -MO=Deparse -e " $key=substr ($input,0, index($input,qq/ /,-1
+));"
$key = substr($input, 0, index($input, ' ', -1));
-e syntax OK
[download]

The qq/ / is seen by Perl as a simple string, just that it was delimited by slashes.