Here is a perl one-liner to pick a fraction x of lines from an input file at random. Note that the resulting file will not have a predetermined but a stochastic number of lines. The example shown will pick a about 30% of all lines

perl -nle 'print $_ unless rand() >0.3' input_file

And here is an example of the expected results:

> for i in 1 2 3 4 5; do head -n 100 input_file | perl -nle 'print $_ unless rand() > 0.3' | wc -l; done
29
28
29
33
33

And if you’d like to pass the fraction of lines desired in the output as a parameter you can do so like this:

perl -nle 'BEGIN {$f=shift} print $_ unless rand() > $f' 0.3 input_file

See perlrun for more details.