Perl Script doesn't update file

Perl Script doesn't update file

I have problem with perl script. It works fine if i don't change
destination path, but when I do in doesn't update my file in that
destination. I got this script from web, but I want to change destination.
Updating works fine when its like this
$rrd_out = '/usr/bin/rrdtool update /home/pi/temperature/rPItemp.rrd
N:$temperature';
But when I change path to this
$rrd_out = '/usr/bin/rrdtool update /dev/shm/rPItemp.rrd N:$temperature';
I have created rrd file in that destination, but don't understand why it's
not updating. Even from terminal I can see that this file executes and
prints values but doesn't update that file in /dev/shm/rPItemp.rrd
Please help me.
Thanks.