The /dev/fd/1 trick works on Linux:

use strict;
use warnings;

sub hello {
  my $filename = shift;
  open my $out, '>', $filename
    or die "Can't write to $filename: $!\n";
  print $out "Hello world!\n";
  close $out or die "Failure closing $filename: $!\n";
  return;
}

hello( '/dev/fd/1' );  # use /dev/fd/1 as the filename for STDOUT
print "Is STDOUT still open?\n";
__END__
Hello world!
Is STDOUT still open?
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