Using DBI instead of shelling out will save your sanity and also that of anyone else who has to maintain your code. Here is an untested example to give you the gist:

#!/usr/bin/env perl 
use strict;
use warnings;
use DBI;

my $dbh = DBI->connect ('DBI:mysql:database=DB1') or die $DBI::errstr;

my $basequery = 'SELECT count(id_) FROM jbpm_taskinstance';
my @where = (
    q#WHERE isopen_ IS TRUE AND actorid_ IS NOT NULL#,
    q#WHERE create_>(SELECT DATE_ADD(NOW(), INTERVAL '-3' HOUR))#,
);

for my $thiswhere (@where) {
    my ($qty) = $dbh->selectrow_array ("$basequery $thiswhere");
    print "Count $thiswhere is $qty\n";
}
$dbh->disconnect;
[download]