is it?

Let's rule out the fact that this deals with symbolic references, strict tells us ...

use strict;
use warnings;

{   no strict "refs";
    my $site = "foo";
    my @{"platforms_$site"} = qw( foo bar );
    my %{"platforms_$site"} = map { $_ => 1 } @{"platforms_$site"};
    }

=>

Can't declare array dereference in "my" at xx.pl line 8, near "} ="
Execution of xx.pl aborted due to compilation errors.
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The difference is in the assignment part. You cannot use my there. When you drop those, you get:

use strict;
use warnings;

{   no strict "refs";
    my $site = "foo";
    @{"platforms_$site"} = qw( foo bar );
    %{"platforms_$site"} = map { $_ => 1 } @{"platforms_$site"};
    }
[download]

Which is still bad coding style, but this works.

It was the best explanation I could find at the moment. But I didn't check with deparse or something.