So, erm, do that. Read perldoc perlopentut and perldoc -f open. Read the docs for Getopt::Std or Getopt::Long. If you're given a filename, open a handle on it; if not, dup STDOUT.

my $fh;
if( $filename ) {
  # open file, put handle in $fh
} else {
  # dup STDOUT, put handle in $fh
}

print $fh "wubba wubba\n";

close( $fh );

exit( 0 );
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Update: expanded the exercise-for-the-reader comments some.