I'm not sure how important the "pricelistname" is to solving the problem... do you really need that at all? Given the nice rephrasing of the problem by Tanktalus, what might work for you is something like this:

my %partprice = ( PN01 => { 2005-01-01 => 29.95,
                            2005-01-05 => 20.00,
                            2005-01-19 => 29,95,
                            2005-03-01 => 33.50 },
                  PN02 => { ... },
                  ...
                 );

# pick a part number and a date of sale:
my $pn = 'PN01';
my $saledate = '2005-02-14';

my $price = getPrice( $saledate, $partprice{$pn} );

# do something with $price ...

sub getPrice
{
    my ( $sdate, $prices ) = @_;  # 2nd param is a hashref

    my @keydates = sort keys %$prices;

    while ( @keydates and $sdate lt $keydates[$#keydates] ) {
        pop @keydates;
    }
    # at this point, either the sale predates the price history (@keyd
+ates is empty)
    # or else the last element of @keydates points to the correct pric
+e for that date

    return ( @keydates ) ? $$prices{pop @keydates} : undef;
}
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I haven't tested that at all (except to confirm that using "pop @array" as the hash key index does work); still I hope the idea is clear enough.