use strict;
use Data::Dumper;
my %result;

{ ### Populate initial tree
  my @prev;

  my $temp=<DATA>;
  chomp $temp;

  foreach my $char (split '', $temp) {
    foreach my $pchar (@prev) {
      $result{$pchar}{$char}{valid} = 1;
    };
    push @prev, $char;
  };
};

{ ### Prune invalid data

  while (<DATA>) {
    chomp;
    my @invalid;
    foreach my $char (split '') {
      foreach my $ichar (@invalid) {
        $result{$char}{$ichar}{valid} = 0;
      };
      push @invalid, $char;
    };
  };
}

my $maxdepthchar;

{ ### Determine depth of each tree
  my $maxdepth;
  foreach my $char (keys %result) {
    getdepth(\%result, $char);
    if ($result{$char}{depth} > $maxdepth) { $maxdepth = $result{$char}{depth}; $maxdepthchar = $char };
  };
}

{ ### Output longest string
  while ($maxdepthchar ne '') {
    print substr($maxdepthchar,0,1);
    $maxdepthchar = $result{$maxdepthchar}{maxdepthchar};
  };
}

sub getdepth{
my ($ref, $char) = @_;
my %result = %{$ref};

my $maxdepth;
my $maxdepthchar;

  if ($result{$char}{depth} eq undef) {
    foreach my $key (keys %{$result{$char}}) {
      if ($result{$char}{$key}{valid}) {
        my $depth = getdepth(\%result, $key);
        if ($depth  > $maxdepth) { $maxdepth = $depth; $maxdepthchar = $key }
      };
    };
    $result{$char}{depth} = $maxdepth+1;
    $result{$char}{maxdepthchar} = $maxdepthchar;
    return $result{$char}{depth};
  } else {
    return $result{$char}{depth};
  };

};

__DATA__
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CXZOL6SUI2WTJ30HF519YPGBRNAK48MQVD7E
T8COSQU6I2FJN40DKL157WVGPYXARZ3MBHE9
KNCWVZDSR5420LP91FIQGB7Y3A6J8MOUXTEH
XF9C4PSDY62TWJ0QBN17IKG3OH8ALVRM5UEZ
D9QCHUSN7TW2YZL0O831FGXIR6JA4P5MVBKE
ZC7ISQUPK6N20OLV4T31G9FRXBAWM5YJHED8
Z3C7SJVODL25TRQ01HPWGNKXB4UA68YMI9EF
BC9OXDHS2FI5Z6U0TYL1VPGQK7ANR38MEWJ4
K4TCQBHS2ZV7FXU0P8R1YGDON3A6JILM9EW5