$ perl -Mstrict -Mwarnings -E '
    my $x = q{1|2|3|||6|7||||};

    {                          
        my $line = $x;
        while($line =~ s/\|\|/\| \|/){}
        say $line;
    }                     
                                           
    {     
        my $line = $x;
        $line =~ s/\|(?=\|)/| /g;
        say $line;
    }    
'
1|2|3| | |6|7| | | |
1|2|3| | |6|7| | | |