$ perl -E'
   rand;
   my $pid = fork;
   srand if $ARGV[0];
   say rand;
   waitpid($pid, 0);
' 0
0.458778299481736
0.458778299481736

$ perl -E'
   rand;
   my $pid = fork;
   srand if $ARGV[0];
   say rand;
   waitpid($pid, 0);
' 1
0.656664192517386
0.153404296449484
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(The seed is determined the first time rand is called, so the problem only manifests itself if rand is called before fork.)