local $ENV{XMLLINT_INDENT} = "\t";
my $linted = "$outfile-lint"; # for readability
unlink $linted;
system( # here is how we avoid the shell: don't use a string. Use a list
  qw(xmllint --format --output), $linted, $outfile
); # don't forget to check return code!
rename $linted, $outfile;