Should 6 be 5 (or even 4)? On my Linux system "mount" returns:

/dev/hda6 on / type ext3 (rw)
0         1  2 3    4    5
[download]

so naturally (split(...))[6] is undef.

Others have commented on your possible misunderstanding of how push works.

Update: I guess it should be 5 since you're assigning to $mnt_acc.