What does the error 'Illegal division by zero' mean?

freak has asked for the wisdom of the Perl Monks concerning the following question:

here is the code :

#!/usr/bin/perl
use Chemistry::Mok;
$code = '/CS/g{ $n++; $l += $match->bond_map(0)->length }
END { printf "Average C-S bond length: %.3f\n", $l/$n; }';

my $mok = Chemistry::Mok->new($code);
$mok->run({ format => mdlmol }, glob("*.mol"));
[download]

here is the error :

Illegal division by zero at (eval 15) line 4.
END failed--call queue aborted.
[download]

NOW, what does it mean?
and how to fix it ?

20040501 Edit by Corion: Added formatting

2004-05-02 Edit by jdporter: Changed title from 'What does this error mean ?'

Back to Seekers of Perl Wisdom