$sql1 = “select * from table1”;
$sql2 = “select * from table2”;

my $table1= $dbh->selectall_hashref($sql1, "primary_key");
my $table2= $dbh->selectall_hashref($sql2, "primary_key");

foreach (keys(%$table1)) {
 $table1_foreign_key = 
  $table2->{$table1->{$_}{foreign_key}}{primary_key};
}