Perl would be overkill.

    #!/usr/bin/sh

    if df -k | grep '\([7-9][0-9]|[0-9][0-9][0-9]\)%' > /dev/null
    then mailx -s "File system(s) full. Clean up your mess." user@foo.
+tla <<'--'
    Yada yada yada

    - The angry sysadmin.
    --
    fi
[download]

This assumes you don't have mount points or volumes that are named "file_at_89%' though. ;-)

Abigail