# Instead of
>perl -w myscript.pl -some -command -line
# you invoke it as
>perl -w myscript.pl -some -command -line >output.xml

##</code><code>##

BEGIN {
  my $outfile = 'output.xml';
  close STDOUT;
  open STDOUT, ">", $outfile
    or die "Couldn't create '$outfile': $!";
};

##</code><code>##

# Instead of
sub frobnitz {
  my ($input) = @_;
  if ($input =~ /foo/) {
    $input =~ s!foo!frobnitz!g;
    print $input;
  };
};

# write:
sub frobnitz {
  my ($input) = @_;
  if ($input =~ /foo/) {
    $input =~ s!foo!frobnitz!g;
    return "<foo>$input</foo>"
  } else {
    return $input;
  };
};

# ...
print frobnitz($in);