This should work:

use Data::Dumper;

my @lines = (
    ['_W9C2JJDCB', 'asdf1', 'zxcv1', ],
    ['_W9C2JJDCB', 'asdf2', 'zxcv2', ],
    ['_W9C2JJDCB', 'asdf3', 'zxcv3', ],
);
my %hash = (
    '_W9C2JJDCB' => [
        '_W92CJJDCB',
        '201200240',
        'TEST: IGNORE',
        'John Doe',
        'Closed',
        'HIP',
        'email@email.com',
        'email2@email.com',
    ],
);

foreach my $key (keys %hash) {
    foreach my $line (@lines) {
        if ($line->[0] eq $key) {
            shift @$line;
            push @{$hash{$key}}, @$line;
        }
    }
}

die Dumper(%hash);
[download]

I don't know what your input data looks like, but may I suggest iterating through @lines instead of keys %hash in the outer loop? This way, you can use the hash's look-up to check if there's a match. Your way is better if there's a massive amount of data in @lines that doesn't have a match in %hash though. Let me know if you'd like some help with the other way!

EDIT: I TAKE BACK WHAT I SAID

You should definitely iterate through @lines first no matter what; it's far more efficient:

foreach my $line (@lines) {
    my $key = shift @$line;
    if (exists($hash{$key})) {
        push @{$hash{$key}}, @$line;
    }
}
[download]