Create /root/free-space-alerts.pl:

#!/usr/bin/perl

# Based on:
#   http://www.cyberciti.biz/tips/howto-write-perl-script-to-monitor-disk-space.html

use strict;
use warnings;
use Filesys::DiskSpace;

if (@ARGV != 3) {
        die "Syntax: $0 <email_address> <path> <min_space_GB>\n";
}

my ($to,$dir,$warning_level) = @ARGV;

my $hostname = `hostname` || 'localhost';
chomp $hostname;

# email setup
my $subject = "$hostname: Low disk space on $dir";

# get df
my ($fs_type, $fs_desc, $used, $avail, $fused, $favail) = df $dir;
$used /= 1024*1024; # to GB
$avail /= 1024*1024; # to GB
my $total = $used + $avail;
my $pct = $avail/$total*100;

# compare
if ($avail < $warning_level) {
        open MAIL, "| /usr/bin/mail $to -s '$subject'" or die "mail: $!\n";
        printf MAIL "WARNING from '$hostname'.\n\nLow disk space on $dir: %.1fGB / %.1fGB free (%.1f%%)\n",$avail,$total,$pct;
        close MAIL;
}

Then create /etc/cron.hourly/free-space/alerts with one line per mountpoint to monitor:

/root/free-space-alerts.pl <YOUR_EMAIL_ADDRESS> <DIRECTORY> <MINIMUM_SPACE_IN_GIGABYTES>
...