$]      The string printed out when you say "perl  -v".   It
             can  be  used  to  determine  at  the beginning of a
             script whether the perl  interpreter  executing  the
             script  is  in the right range of versions.  If used
             in  a  numeric  context,  returns  the   version   +
             patchlevel / 1000.  Example:

                  # see if getc is available
                     ($version,$patchlevel) =
                        $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
                     print STDERR "(No filename completion available.)\n"
                        if $version * 1000 + $patchlevel < 2016;

             or, used numerically,

                  warn "No checksumming!\n" if $] < 3.019;

             (Mnemonic: Is this version  of  perl  in  the  right
             bracket?)