The issue seems to be that @feed can be set to bogus sequences of words. If the input stream is obtained from the following __DATA__ section:

__DATA__
the quick brown fox
brown fox jumps
jumps
the quick fox jumps
[download]

then in the second itteration of the while($cyc_count2 <= $keysout){ loop @feed contains qw(quick brown brown) and the hash lookup fails because no such sequence existed in the original text.

Fixing the issue is left as an exercise for the reader ;).