my %players;
for (<DATA>) {
    chomp;
    my ($team) = m!^ (.+?) = (?<left>.+?) \s*/\s* (?<right>.+) $!x
        or die "Line `$_' doesn't match";

    for my $side (qw<left right>) {
        $players{$_}{$side} = $team for split m!\s*[,/]\s*!, $+{$side};
    }
}

# Remove players only on one team
delete @players{ grep { keys %{$players{$_}} == 1 } keys %players };

local $" = ' AND ';
print "$_ IS IN @{$players{$_}}{qw<left right>}\n" for keys %players;

##</code><code>##

my %players;
for (<DATA>) {
    chomp;
    my ($team,$players) = split /=/;
    push @{$players{$_}}, $team for split m!\s*[,/]\s*!, $players;
}

delete @players{ grep { @{$players{$_}} == 1 } keys %players };
    
local $" = ' AND ';
print "$_ IS IN @{$players{$_}}\n" for keys %players;