The 2-arg open understands - to mean STDIN or STDOUT.

# XXX Unable to handle paths starting with a space or a
#     file named "-". The workaround is to prepend "./".

my $file = @ARGV ? shift(@ARGV) : '-';
open(my $fh, "> $file")
   or die("Unable to open output file \"$file\": $!\n");

print $fh "this is a test\n";
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Otherwise, you're stuck with an if statements. You could combine the two if statements into one, though.

Update: Added code.