As others have suggested, if you want a dictionary word, you need a dictionary. The solutions I've seen so far, however, have involved loading the whole dictionary into memory to select your word. I think you could do this without loading the whole dictionary, however. See How do I pick a random line from a file? for the basic algorithm, which could be adapted to apply only to words of a certain length.

Update: I just couldn't resist writing it myself.

use strict;
use warnings;

my $desired_length = 8;
my $selected_word;
my $candidate_count = 0;

open my $dict_fh, '<', '/usr/share/dict/words'
    or die "Can't read '/usr/share/dict/words': $!\n";
WORD:
while ( my $word = <$dict_fh> ) {
    chomp $word;
    next WORD if length $word != $desired_length;
    $selected_word = $word if rand ++$candidate_count < 1;
}

print "Selected '$selected_word' out of $candidate_count candidates.\n
+";
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