my @mapped_prizes = map +{ $month{(split(/-| /,$_->{'Timestamp'}))[1]} => $_ },  @prizes;

##</code><code>##

sub month_from_datetime {
    my ( $datetime ) = @_;
    ( split m{-| }, $datetime )[1]
}

my %month_name = (
    # ...
);

my %mapped_prizes = map {
    my $month_num = month_from_datetime $_->{'Timestamp'};
    $month_name{ $month_num } => $_;
}, @prizes;