What do you mean by "not working"? It works quite fine for me.

use Data::Dumper;
print Dumper \%hash_mnt;
__END__
$VAR1 = {
        '/proc/bus/usb' => [ ['usbfs'],     [undef] ],
        '/sys'          => [ ['none'],      [undef] ],
        '/'             => [ ['/dev/hda1'], [undef] ],
        '/dev/pts'      => [ ['none'],      [undef] ],
        '/tmp'          => [ ['/dev/hda5'], [undef] ],
        '/proc'         => [ ['proc'],      [undef] ],
        '/home'         => [ ['/dev/hda6'], [undef] ]
};
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Do you mean that this structure is not what you wanted? It seems most sensible that you'd want a hash of arrays instead of the hash of arrays of arrays. Maybe like so?

foreach $mount(@Mounts){
    ($mnt_key,$mnt_val,$mnt_acc)=(split(/\s+/,$mount))[2,0,6];
    $hash_mnt{$mnt_key} = [ mnt_val, $mnt_acc ];
}

use Data::Dumper;
print Dumper \%hash_mnt;
__END__
$VAR1 = {
        '/proc/bus/usb' => [ 'usbfs',     undef ],
        '/sys'          => [ 'none',      undef ],
        '/'             => [ '/dev/hda1', undef ],
        '/dev/pts'      => [ 'none',      undef ],
        '/tmp'          => [ '/dev/hda5', undef ],
        '/proc'         => [ 'proc',      undef ],
        '/home'         => [ '/dev/hda6', undef ]
};
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