A Perl quote-like operator used to delimit a regular expression (RE) like "?FOO?" that matches FOO at most once. The normal "/FOO/" form of regular expression will match FOO any number of times. The "??" operator will match again after a call to the "reset" operator.
The operator is usually referred to as "??" but, taken literally, an empty RE like this (or "//") actually means to re-use the last successfully matched regular expression or, if there was none, empty string (which will always match).
Unix manual page: perlop(1).