You say don't want to replace the .* at the end with \S+, because the last column could contain arbitrary columns. However, that's not a problem for the first column, so you can replace the .+:

     my ($ptid, $total, $used, $avail, $pct, $mp) = $element =~
       m!^(/dev/\S+)           # which partition $ptid
           \s+([\d.]+[MGK])    # total size of parition $total
           \s+([\d.]+[MGK])    # used space $used
           \s+([\d.]+[MGK])    # available space $avail
           \s+(\d{1,3})%       # percent usage $pct
           \s+(.*)$!x;         # mounting point $mp
[download]

This will save the regex engine from having to match to the end of the line and then backtrack all the way back to the first column. I also changed 0-9 to \d in the character classes, and changed \d{2} to \d{1,3}, assuming you don't really want to ignore devices that are 100% full or less than 10% full.

BTW, your regex isn't compatible across Unix platforms:

Filesystem             Type  blocks     use     avail  %use Mounted on
/dev/root               xfs 16718720 11050536  5668184  67  /
/dev/dsk/dks1d6s0       xfs  8758624  1968528  6790096  23  /usr/darmo
+k
AFS                     afs 14400000        0 14400000   0  /afs
[download]

Anyway, I think tilly's and lemming's suggestion of using split with a limit is the best approach.