I recommend one of the following. (All the code below are UNTESTED and may contain errors.)

• If the output data is short, just collect it to a string, eg.

  sub foo {
      my $o = "";
      $o .= "blah\n";
      $o;
  }
  if ($to_file == 0) {
      print foo();
  } else {
      print MYFILE foo();
  }
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• Use the default filehandle you select, eg.

  sub foo {
      print "blah\n";
  }
  my $OLDH;
  if ($to_file) {
      $OLDH = select MYFILE;
  }
  foo();
  $OLDH and select $OLDH;
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• Pass a filehandle as an argument to the function and print to that, eg.

  sub foo {
      my($O) = @_;
      print $O "blah\n";
  }
  foo($to_file ? *MYFILE : *STDOUT);
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• Pass a callback sub as an argument to the function that's called for each line.

  sub foo {
      my($P) = @_;
      &$P("blah\n");
  }
  foo($to_file ? sub { print MYFILE @_ } : sub { print @_ });
  # or foo(sub { if ($to_file) { print MYFILE @_ } else { print @_ } });
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• Same as above but with a global variable or global sub instead of argument, eg one of.

  our $O;
  sub foo {
      print $O "blah\n";
  }
  $O = $to_file ? *MYFILE : *STDOUT;
  foo();
  # OR
  sub foo {
      print O "blah\n";
  }
  *O = $to_file ? *MYFILE{IO} : *STDOUT{IO};
  # (the {IO} is not strictly needed but this way we avoid overwriting $
  +O etc)
  foo();
  # or same as above but like
  open O, ">&=", ($to_file ? *MYFILE : *STDOUT);
  # or you could even use *MYFILE instead of *O
  # OR
  sub foo {
      my($P) = @_;
      pri("blah\n");
  }
  sub pri {
      if ($to_file) { print MYFILE @_ } else { print @_ }
  }
  [download]