First all, apologies for ruining some nice code. I have added a print statement and 4 comment points (A .. D). 
#!/usr/bin/perl -w
use strict;
use warnings;

my %piles = (A => [reverse 1..3], B => [], C => []);

dumpPiles ();
hanoi (scalar @{$piles{A}}, keys %piles); #A

sub hanoi {
   my ($n, $start, $end, $extra, $recursion) = @_;
   print "Hanoi: Disk $n\n";
   if ($n == 1) { #B
        report ($start, $end, $recursion);
   } else {
        hanoi($n-1, $start, $extra, $end); #C
        report ($start, $end, $recursion);
        hanoi($n-1, $extra, $end, $start); #D
   }
}

sub report {
    my ($start, $end) = @_;
    my $disk = pop @{$piles{$start}};

    print "Moved disk $disk from $start to $end.\n";
    push @{$piles{$end}}, $disk;
    dumpPiles ();
}

sub dumpPiles {
    for my $pile (sort keys %piles) {
        print "$pile: @{$piles{$pile}}\n";
    }
}

[download]

The first hanoi call happens at point A, where the first argument (which disk to move) is equal the number of discs on pile A; namely '3'. We then move into the hanoi iterative method where the logical statement at B is false and we move to point C.

At point C we call hanoi again (note we have not called report yet, but the first argument has been decremented to '2'. This means we arrive at point B for the second time and the logical statement is still false.

We arrive at point C again, calling hanoi on disk number 1. Now the statement at B is true and the iterations begin to unravel.

Perhaps, together with the additional print statement in the code, this will help to explain things?

-=( Graq )=-

In reply to Re^3: cannot follow hanoi subroutine by graq
in thread cannot follow hanoi subroutine by convenientstore

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