sub buildtree {
        my($f, @a) = @_;
        local $_;

        @a = ((sort $f @a), '');
        $_ = join '=>[0,{', @a;
        chop;
        $_ = '(' . $_ . '0' . ']}' x $#a;
        s/}$/)/;
        eval;
}
##</code><code>##

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

sub buildtree {
        my($f, @a) = @_;
        local $_;

        @a = ((sort $f @a), '');
        $_ = join '=>[0,{', @a;
        chop;
        $_ = '(' . $_ . '0' . ']}' x $#a;
        s/}$/)/;
        eval;
}

my @array = ( 6, 1, 2, 3, 4, 5 );
my $func = sub($$) { $_[0] <=> $_[1] };

my %tree = buildtree( $func, @array );

print Dumper \%tree;

##</code><code>##

        #        1         2         3         4
        #23456789012345678901234567890123456789012345
        my$f=shift;@_=((sort{&$f($a,$b)}@_),'');$_=
        join'=>[0,{',@_;chop;%{eval"{$_ 0".']}'x$#_};