You're relatively close, but remember hashes can only have one value per key name. What you want is to push the new hash reference into an array under the top level key name. Here's an example:

while (<DATA>){
    my @ju = split /\s+/, $_;

    push @{ $self->{code}{$ju[0]} },
        {
            event => $ju[1],
            month => $ju[2],
            am => $ju[3],
            pm => $ju[4],
        };
}

print Dumper $self->{code};

__END__

# trimmed down example output

$VAR1 = {
          'RF858' => [
                       {
                         'am' => '7.22',
                         'event' => 'U888',
                         'pm' => '4.20',
                         'month' => 'Dec'
                       }
                     ],
          'VS524' => [
                       {
                         'am' => '5.94',
                         'month' => 'Jan',
                         'pm' => '9.46',
                         'event' => 'U509'
                       },
                       {
                         'am' => '5.22',
                         'month' => 'Aug',
                         'pm' => '1.78',
                         'event' => 'U14'
                       }
                     ],
        };
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Here's one way that describes how to access all portions of the struct

for my $top_key (keys %{ $self->{code} }){
    print "$top_key\n";
    for my $href (@{ $self->{code}{$top_key} }){
        for (keys %$href){
            print "\t$_ => $href->{$_}\n";
        }
        print "\n";
    }
}
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