use Modern::Perl;

my $sum_of_used;
{
    no warnings qw/numeric uninitialized/;
    $sum_of_used += (split /\s+/)[2] while <DATA>;
}
say $sum_of_used;

__DATA__
<df -k>
Filesystem               total       used      avail capacity  Mounted on
/vol/vol0/          142606336KB  5274244KB 137332092KB      4%  /vol/vol0/
/vol/vol0/.snapshot 35651584KB   923524KB 34728060KB       3%  /vol/vol0/.snapshot
/vol/vobstore/      734003200KB  6335232KB 727667968KB       1%  /vol/vobstore/
/vol/vobstore/.snapshot 183500800KB   260200KB 183240600KB       0%  /vol/vobstore/.snapshot
/vol/ccase/         125829120KB 21814420KB 104014700KB      17%  /vol/ccase/
/vol/ccase/.snapshot 31457280KB    90572KB 31366708KB       0%  /vol/ccase/.snapshot
/vol/viewstore/     943718400KB  6492604KB 937225796KB       1%  /vol/viewstore/
/vol/viewstore/.snapshot 235929600KB   137500KB 235792100KB       0%  /vol/viewstore/.snapshot
/vol/viewstore_win/ 67108864KB      480KB 67108384KB       0%  /vol/viewstore_win/
/vol/viewstore_win/.snapshot 16777216KB     2372KB 16774844KB       0%  /vol/viewstore_win/.snapshot
</df -k>