#!/usr/bin/perl

use strict;
use warnings;

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

##</code><code>##

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 at hanoi.pl line 33.