Why not think of STDIN, STDOUT and STDERR as a kind of constants? And really they look like nothing different, do they? Perl constants are used to be implemented as named subroutines with a zero-argument prototype, returning a filehandle reference.

Unfortunately however, passing an actual user-defined constant filehandle to print() will work only when wrapped in curlies, likewise would any other expression returning a filehandle:

use strict;
sub FH () { open my $fh, '>', '/tmp/testfile.txt'; $fh }
eval q{ print FH "Hello world!\n"; } or warn "Doesn't work - $@";
print {FH} "This works";
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The curlies are recommended best practice for every time you must use type-globs, cf. Conway(2006). Think of it as type-glob derefencing with the star sigil omitted again.

Nowadays you should make lexical filehandles, how this is done has been already outlined in the thread.