Like I said, probably a step too far, but I was trying to stay true to the Lisp original. If the intent is to completely Perlify the algorithm, then you probably wouldn't use the listp() abstraction (and it certainly wouldn't be called that:).

If Perl's if then elsif else construct worked like those in Haskell or SQL, you could do

return if <cond> then <value> elsif <cond2> then <val2> else <val3>;
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The nearest I could get is the nested ternaries.

I also tried to code the cond macro, and got pretty close. It follows all the right paths, but implementing rewrite is much harder, and the values don't filter up without that.

#! perl -slw
use strict;

my %dict = (
  sentence     => [ [ qw/ noun_phrase verb_phrase / ] ],
  noun_phrase  => [ [ qw/ Article Noun / ] ],
  verb_phrase  => [ [ qw/ Verb noun_phrase / ] ],
  Article      => [ qw/ the a /],
  Noun         => [ qw/ man ball woman table/ ],
  Verb         => [ qw/ hit took saw liked / ]
);


sub rand_elt {
    return @{ $_[ 0 ] }[ rand @{ $_[ 0 ] } ];
}

sub listp {
    return ref $_[0] eq "ARRAY";
}

sub cond;
sub cond {
    return shift->() || cond @_;
}

sub generate;
sub generate {
    my $phrase = shift;
    warn $phrase.$/;

    return cond
        sub{ 
            warn 'A'.$/; 
            return listp( $phrase )        
                ? map{ generate $_ } @{ $phrase }    
                : () 
        },
        sub{ 
            warn 'B'.$/; 
            return exists $dict{ $phrase } 
                ? generate rand_elt $dict{ $phrase } 
                : () 
        },
        sub{
            warn 'C'.$/;
            return 1                       
                ? $phrase  
                : () 
        };
}
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