sub gen_iterator {
    my @initial_info = @_;
    my ($current_state, $done);
    return sub {
        # code to calculate $next_state or $done;
        return undef if $done;
        return $current_state = $next_state;   
    };
}

##</code><code>##

my $next = gen_iterator( 42 );
while ( my $item = $next->() ) {
    print "$item\n";
}

##</code><code>##

my @DNA = qw/A C T G/;
my $seq = gen_permutate(14, @DNA);

while ( my $strand = $seq->() ) {
    print "$strand\n";
}
sub gen_permutate {
    my ($max, @list) = @_;
    my @curr;
    return sub {
        if ( (join '', map { $list[ $_ ] } @curr) eq $list[ -1 ] x @curr ) {
            @curr = (0) x (@curr + 1);
        }
        else {
            my $pos = @curr;
            while ( --$pos > -1 ) {
                ++$curr[ $pos ], last if $curr[ $pos ] < $#list;
                $curr[ $pos ] = 0;
            }
        }
        return undef if @curr > $max;
        return join '', map { $list[ $_ ] } @curr;
    };
}

##</code><code>##

my $start = $ARGV[0] || 999999;
my $next_id = gen_id( $start );
print $next_id->(), "\n" for 1 .. 10;  # Next 10 IDs

sub gen_id {
    my $curr = shift;
    return sub {
        0 while ! is_valid( ++$curr );
        return $curr;
    };
}
sub is_valid {
    my ($num, $chk) = (shift, '');
    my $tot;
    for ( 0 .. length($num) - 1 ) {
        my $dig = substr($num, $_, 1);
        $_ % 2 ? ($chk .= $dig * 2) : ($tot += $dig);
    }
    $tot += $_ for split //, $chk;
    return $tot % 10 == 0 ? 1 : 0;
}

##</code><code>##

my $next_file = rotate( qw/FileA FileB FileC/ );
print $next_file->(), "\n" for 1 .. 10;

sub rotate {
    my @list = @_;
    my $index = -1;
    return sub {
        $index++;
        $index = 0 if $index > $#list;
        return $list[ $index ];
    };
}

##</code><code>##

while ( my $pass = $next_pw->() ) {
    if ( unlock( $pass ) ) {
        print "$pass\n";
        last;
    }
}
sub fix_size_perm {
    my ($size, @list) = @_;
    my @curr = (0) x ($size - 1);
    push @curr, -1;
    return sub {
        if ( (join '', map { $list[ $_ ] } @curr) eq $list[ -1 ] x @curr ) {
            @curr = (0) x (@curr + 1);
        }
        else {
            my $pos = @curr;
            while ( --$pos > -1 ) {
                ++$curr[ $pos ], last if $curr[ $pos ] < $#list;
                $curr[ $pos ] = 0;
            }
        }
        return undef if @curr > $size;
        return join '', map { $list[ $_ ] } @curr;
    };
}
sub unlock { $_[0] eq 'john' }

##</code><code>##

my @list = map { my @lines = <$_>; \@lines } @f_handles;
# becomes
my $next = gen_fh_iterator( \@f_handles );
while ( my $lines = $next->() ) {
    print @$lines, "\n";
}
sub gen_fh_iterator {
    my $f_handle = shift;
    my $index = -1;
    return sub {
        $index++;         
        return undef if $index > $#f_handle;
        my @lines = <$f_handle->[$index]>;
        return \@lines;
    };
}

##</code><code>##

sub infinite_evens {
    my $curr = shift;
    $curr++ if $curr % 2;
    my $next;
    return sub {
        if ( ! defined $next ) { return $next = $curr }
        return $next += 2;
    };
}
my $next_even = infinite_evens( 41 );
print $next_even->(), "\n" while 1;