Abstracting the hash helps play with different implementations. A pack/unpack and an array variant (commented out) are shown:

use warnings;
use strict;

my @large_list = (
    {key => 1, cond => 1},
    {key => 2, cond => 0},
    {key => 3, cond => 0},
    {key => 3, cond => 0},
    {key => 1, cond => 0},
    );

my $record = bless {};

for my $item (@large_list) {
    $record->add ($item);
}

for (sort keys %$record) {
    print "Item $_ count ", $record->getCount ($_), ', flag ', $record
+->getFlag ($_), "\n";
}


sub add {
    my ($self, $item) = @_;
    my $key = $item->{key};

    if (! exists $self->{$key}) {
        $self->{$key} = pack ('NN', 1, $item->{cond});
    } else {
        my ($count, $flag) = unpack ('NN', $self->{$key});
        $self->{$key} = pack ('NN', $count + 1, $flag || $item->{cond}
+);
    }
}


sub getFlag {
    return (unpack ('NN', $_[0]->{$_[1]}))[1];
}


sub getCount {
    return (unpack ('NN', $_[0]->{$_[1]}))[0];
}


#sub add {
#    my ($self, $item) = @_;
#    my $key = $item->{key};
#
#    if (! exists $self->{$key}) {
#        $self->{$key} = [1, $item->{cond}]
#    } else {
#        $self->{$key}[0]++;
#        $self->{$key}[1] ||= $item->{cond};
#    }
#}
#
#
#sub getFlag {
#    return $_[0]->{$_[1]}[1];
#}
#
#
#sub getCount {
#    return $_[0]->{$_[1]}[0];
#}
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Prints:

Item 1 count 2, flag 1
Item 2 count 1, flag 0
Item 3 count 2, flag 0
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