#! perl -slw
use strict;

sub ATOa{ map{ eval '\substr( $_[0], $_ )' } 0 .. length( $_[0] ); }

my @Aoa = sort{ $$a cmp $$b } ATOa $ARGV[0];

#print $$_ for @Aoa;

my $max = '';

for my $i ( 0 .. ($#Aoa-1) ) {
	my $p = 0;
	++$p while substr( ${$Aoa[ $i   ]}, 0, $p )
	        eq substr( ${$Aoa[ $i+1 ]}, 0, $p );

	$max = substr( ${$Aoa[ $i ]}, 0, $p-1 )
	    if length $max < $p;
}

print "The longest common substring in\n'$ARGV[0]'\n is '$max'";

__DATA__
P:\test>test "the infernal invention was the source or eternal facination for the qualified professional and enthusiastic amateur alike"
The longest common substring in
'the infernal invention was the source of eternal facination for the qualified professional and enthusiastic amateur alike'
 is 'ernal '

P:\test>test "Four and twenty virgins came down from Inverness. And when they went back again there were four and twenty less"
The longest common substring in
'Four and twenty virgins came down from Inverness. And when they went back again there were four and twenty less'
 is 'our and twenty '