Here's one way (untested tested):

use strict;
use warnings;

use Data::Dumper;

my %disks;

{
    local $/ = "};";
    while( my $record = <DATA> ) {
        chomp $record;
        my %attribs;
        foreach my $line ( split /\n/, $record ) {
            next unless $line =~ m/=.+;/; 
            chomp $line;
            $line =~ s/^\s+//;            
            $line =~ s/\s*;\s*$//;
            my( $key, $value ) = split /\s*=\s*/, $line;
            $value =~ s/"//g;
            $attribs{$key} = $value; 
        }
        next if not exists $attribs{'Name'}; # Just in case. ;) 
        $disks{ $attribs{ 'Name' } } = { %attribs }; 
    }
}
print Dumper \%disks;

__DATA__
instance of Win32_LogicalDisk
{
        FreeSpace = "114151464960";
        Name = "C:";
        Size = "160031014912";
};

instance of Win32_LogicalDisk
{
        FreeSpace = "5515554816";
        Name = "D:";
        Size = "203921108992";
};

instance of Win32_LogicalDisk
{
        FreeSpace = "43128733696";
        Name = "H:";
        Size = "400086708224";
};
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Updated: Tweaked code to provide HoH rather than AoH.