Hello dipit,

Welcome to the Monastery. It is really difficult for us to replicate your problem without having input data (sample of table).

Having said that, I have found MySQL select statements in the past failing because of not using back ticks (``). So try something like this:

Give it a try and let us know if it worked.

Update: I have found really useful the phpMyAdmin it helps me to check my requests before applying them.

Update2: In case the SELECT statement is correct run something like this as fellow monk talexb proposed:

eval {      ### Catch _any_ kind of failures from the code within

    ### Enable auto-error checking on the database handle
    $dbh->{RaiseError} = 1;

    ### Prepare a SQL statement for execution
    my $sth = $dbh->prepare( "select distinct `FID_CUST` from `session
+` where `DAT_END` between `$key` and `$hash{$key}`" );

    while (1) {
    ### Execute the statement in the database
    $sth->execute();

    ### Retrieve the returned rows of data
    while ( my @row = $sth->fetchrow_array() ) {
        print "Row: @row\n";
    }
    }
};
warn "Monitoring aborted by error: $@\n" if $@;
[download]

Looking forward to your update, BR.