You can get away with using the Lvalues from substr in the array as suggested by Zaxo above, but you have to jump through a few hoops to prevent the problem I alluded to.

Basically, there seems to be a single slot allocated for all the Lvalues generated by any given statement that includes a substr. That's a clumsy way to put it, but I can't think of a better way to say it.

To work around the limitation, it is necessary to use a 'different' substr each time, hence the need for the eval in this code. The downside is that the eval markly reduces the efficiency. The algorithm works fine though and this implementation shoudl be a bit quicker than yours.

#! 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 facinat
+ion for the qualified professional and enthusiastic amateur alike"
The longest common substring in
'the infernal invention was the source of eternal facination for the q
+ualified professional and enthusiastic amateur alike'
 is 'ernal '

P:\test>test "Four and twenty virgins came down from Inverness. And wh
+en 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 '
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