Try:

foreach my $href ( @{$refdata->{'Map'}} ){
    print $href->{'MapId'} ."\n";
}
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$refdata->{Map} is an arrayref, to get the array contents you need to stick an '@' on the front and add braces to specify what the '@' applies to. Without the braces it would try to treat $refdata as an arrayref - which it is not.

I'm parsing an XML Document into a perl data structure using XML::Simple with the default (Read: No) options

I can't recommend that :-) You should at least use:

my $refdata = XMLin('C:/myxmltest.xml', forcearray => [ 'Map' ]);
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That way your code won't break if there's only one 'Map' element.