Help needed with perl crypt function

Question

I am trying to generate a unique string/id given another relatively large string(consisting of a directory path name), thought of using crypt function. However, it's not working as expected, most probably due to my inability to understand.

here the code & output:

#!/usr/bin/perl 

print "Enter a string:"; 
chomp(my $string = <STDIN>); 

my $encrypted_string = crypt($string,'di'); 

print "\n  the encrypted string is:$encrypted_string";

output:

$ perl crypt_test
 Enter a string:abcdefghi

 the encrypted string is:dipcn0ADeg0Jc
$
$ perl crypt_test
 Enter a string:abcdefgh

 the encrypted string is:dipcn0ADeg0Jc
$
$
$ perl crypt_test
 Enter a string:abcde

 the encrypted string is:diGyhSp4Yvj4M
$

I couldn't understand why it returned the same encrypted string for the first two strings and differed for the third one. Note that salt is same for all.