AA
AB
ABC
ABCDE

##</code><code>##

AA
AB
AB
AB

##</code><code>##

AA
AB
ABC
ABCD

##</code><code>##

#!/usr/bin/perl -lw

use strict;

chomp( my @lines = <DATA> );

# find shortest line
my @foo = sort { length $a <=> length $b } @lines;
my $diff = $foo[0];


LINES:
  for (@lines) {
    # the longest common substring can only be as long
    # as the shortest line
    my $line = substr $_, 0, length $diff;
    while( $line ) {
      if ( $line eq $diff ) {
        $diff = $line;
        next LINES;
      }
      else {
        # reduce current line and recent common string by one
        $line = substr $line, 0, -1;
        $diff = substr $diff, 0, -1;
      }
    }
  }

for( @lines ) {
  print substr $_, 0, length( $diff ) + 1;
}

__DATA__
AA
AB
ABC
ABCDE