get the error message - Can't call method "avail" on an undefined value.

Ronnie,
To paraphraase the man page for this module:

#!/usr/bin/perl -w
######################################
 
use strict;
use warnings;
use Filesys::DiskFree;
  
my $handle = new Filesys::DiskFree; # ASSIGN HANDLE!
$handle->df();
print "The root device is ".$handle->device("/")."\n";
print "It has ".$handle->avail("/")." bytes available\n";
print "It has ".$handle->total("/")." bytes total\n";
print "It has ".$handle->used("/")." bytes used\n";
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Notice the assignment of the variable $handle. This is important. In order to reference something it has to exist. Now when I run the code on my Linux box on my desk at work I get:

The root device is /dev/hda1
It has 669364224 bytes available
It has 9253654528 bytes total
It has 8114221056 bytes used
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Hope this helps.