From the docs for print:

Note that if you're storing FILEHANDLEs in an array, or if you're using any other expression more complex than a scalar variable to retrieve it, you will have to use a block returning the filehandle value instead:

I believe the braces define a block of code, and I believe $_[0] qualifies as an 'expression more complex than a scalar variable'. If you refactor the sub like this, then the braces around the FILEHANDLE are not needed:

use warnings; use strict;
sub print_to {
    my ($fh, $str) = @_;
    print $fh $str;
}
print_to (*STDOUT, "test stdout");
print_to (*STDERR, "test stderr");
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