It is true that key collision is possible. I needed to knock something together quickly (hence this is v0.1 :) ) and seeing that:

perl -e 'print rand, "\n"';
0.315919870045036
[download]

and that I only had 9000 lines in my file, it was thought the probability of key collision was small.

I actually tested it on the file about 10 times and saw that the number of lines in the randomised file was equal to the original file.

No doubt I will modify this one day when I feel like it...

Thanks to all those who replied,

== fx, Infinity is Colourless