I've been reading a book "Higher-Oder Perl" by Mark Jason Dominus and I came to one of his examples on recursion but I'm having problems with it and I can't seem to see what I've typed wrong.

When i try and run this code it fails to compile. it seems from the error I'm getting that I'm trying to pass a null value as my code reference. I can't seem to see why this is though. code some one please take a look and see where my mistake is. i thought I copied every thing out of the book verbatim and it looks the same. I need a fresh set of eyes.

I've checked his website for this book and he doesn't have an example of this version it's just he suggest the passing subroutine as a suggestion in the book.

#!/usr/bin/perl

use strict;
use warnings;

my $disk = 20;
my @position = ( '', ('A') x $disk );    # disk are all initially on p
+eg A

sub check_move
{
    my $i;
    my ( $disk, $start, $end ) = @_;

    if ( $disk < 1 or $disk > $#position )
    {
        die "Bad disk number $disk. should be 1..$#position.\n";
    }
}

sub hprint
{
    my ( $disk, $start, $end ) = @_;
    print "Move disk #$disk from $start to $end\n";
}

sub hanoi
{
    my ( $n, $start, $end, $extra, $move_disk ) = @_;

    if ( $n == 1 )
    {
        $move_disk->( 1, $start, $end );
    }
    else
    {
        hanoi( $n - 1, $start, $extra, $end );
        $move_disk->( $n, $start, $end );
        hanoi( $n - 1, $extra, $end, $start );
    }
}

hanoi( $disk, 'A', 'B', 'C', \&check_move );
hanoi( $disk, 'A', 'B', 'C', \&hprint );

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the error I'm getting is: 
Use of uninitialized value in subroutine entry at hanoi.pl line 33.
Can't use string ("") as a subroutine ref while "strict refs" in use a
+t hanoi.pl line 33.

[download]

In reply to passing subroutine references by joe76

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