use warnings;
use strict;

my %ch;
my $part;
my $lines;

while (<DATA>) {
    chomp;
    $part="";
 
    map { $part .= $_;
          print $part."\n";
          $ch{$part}++;
         } split (//);
    $lines ++;
}

my $max;
foreach  (keys (%ch) ) {
    if ( ($ch{$_} == $lines) and (length($_) > length($max)) ) {
        $max = $_;
    }
}

print $max;

__DATA__
abcdefg
abcd
abcd grtf
abcd abd
abcd daf