You're wanting to check out the DBI module to get your connection to the database. Here is a simple way of going about it; I'm sure you can do the differencing within the SQL, but I never did go and learn SQL functions :). So this is one way of going about it:

=for SQL Table
------------------------------------
| theIDNum | theDate    | thePrice |
------------------------------------
| ax1      | 2004-12-01 | 300.00   |
| ax2      | 2004-12-01 | 500.00   |
| ax1      | 2004-12-02 | 200.00   |
| ax1      | 2004-12-03 | 100.00   |
| ax3      | 2004-12-01 | 300.00   |
| ax2      | 2004-12-01 | 400.00   |
------------------------------------
=cut

use strict;
use DBI;

my $dbh = DBI->connect('dbi:mysql:database', 'u', 'p', {
    RaiseError => 1, AutoCommit => 1
} );

my $rows = $dbh->selectall_arrayref(
    'SELECT thePrice FROM theTable ORDER BY ' .
    'theDate DESC, thePrice LIMIT 2'
);

my $diff = abs( $rows->[0][0] - $rows->[1][0] );

printf("%s%.2f\n", 'Difference of $', $diff);

__END__

Difference of $50.00
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