AoC day 22 (pt2), Perl

#!/usr/bin/perl

use strict;
use warnings;

$| = 1;
$/ = '';

my $Game = 0;

# Read decks:
my @deck;
foreach my $p (0 .. 1) {
    $_ = <>;
    $deck[$p]->@* = split( "\n" );
    shift @{$deck[$p]};
}

sub score_deck {
    my $score = 0;
    my $i = 1;
    while (@_) {
        $score += ($i * pop @_);
        $i++;
    }
    return ($score);
}

sub make_state {
    my ($pd0, $pd1) = @_;
    return (join( ',', @$pd0 ) . ':' . join( ',', @$pd1 ));
}

sub recurse_game {
    my ($depth, $d0, $d1 ) = @_;

    my %state;

    $Game++;
    print "Starting game #$Game at depth $depth\r"  if ($Game % 100 == 0);

    while (@$d0 && @$d1) {
        # loop detection:
        my $curr_state = &make_state( $d0, $d1 );
        if (exists $state{$curr_state}) {
            return (($depth > 0) ? 0 : &score_deck( @$d0 ));
        }
        $state{$curr_state}++;

        # draw cards:
        my ($c0, $c1) = (shift @$d0, shift @$d1);

        my $winner;
        if ($c0 <= scalar(@$d0) && $c1 <= scalar(@$d1)) {
            my @new_d0 = map { $d0->[$_] } (0 .. $c0 - 1);
            my @new_d1 = map { $d1->[$_] } (0 .. $c1 - 1);
            $winner = &recurse_game( $depth + 1, \@new_d0, \@new_d1 );
        } elsif ($c0 > $c1) {
            $winner = 0;
        } else {
            $winner = 1;
        }

        # winner takes cards
        if ($winner == 0) {
            push( @$d0, ($c0, $c1) );
        } else {
            push( @$d1, ($c1, $c0) );
        }
    }

    # sub games return winner, main game returns score
    if ($depth > 0) {
        return ((@$d0) ? 0 : 1);
    } else {
        return (&score_deck( @$d0 || @$d1 ));
    }
}

print "\nPart 2: ", &recurse_game( 0, $deck[0], $deck[1] ), "\n";