If you want to match tree, but not tree2 (for example), try:
next unless /^tree[ ]/;
instead of
next unless /^tree/;

Replaces 5th field:

s/^(\S+[ ]+\S+[ ]+\S+[ ]+\S+[ ]+)(\S+[ ]+)/
   my $s = int(rand($2));
   $1 . $s . (' ' x (length($2) - length($s)))
/e;
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Update: Added missing "$1 . ", as noticed by Animator. Removed unecesary if. Tested.