A block is 'code' in this case code that returns a file handle - see how perl deparse's the above code and puts a semicolon before the right curly brace in the code below:
$ perl -MO=Deparse fh.pl

die "Ouch. $!" unless open my $fh, '>', 'foo.txt';
print {$fh;} "hi!\n";
close $fh;
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fh.pl syntax OK
(there are probably other methods that you can use to have perl show you what it is "thinking")
perldoc -f print does mention the blocks

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:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";
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