Given that Time::HiRes is a core module I think one can assume it is generally well supported, so using select(undef, undef, undef, $n) seems to me to be unnecessarily baroque.

You can, of course, use Time::Hires qw(sleep) and magically transform all the sleep in the current package... Or go a stage further and:

  use Time::HiRes () ;
  BEGIN { *CORE::GLOBAL::time = \&Time::HiRes::time ; } ;
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and drag your entire program into the 21st Century, if you're feely lucky, modules-wise !

However, the limitations of sleep are so well known, that overriding it will probably confuse -- so I favour a small utility module:

  package Htime ;

  use Exporter qw(import) ;
  our @EXPORT = qw(htime hsleep halarm) ;

  use Time::HiRes () ;

  *htime  = \&Time::HiRes::time ;
  *hsleep = \&Time::HiRes::sleep ;
  *halarm = \&Time::HiRes::alarm ;
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...and one can add a little code to check that a given minimum resolution is supported locally. It's then clear that where hsleep is used it isn't standard sleep -- and without all the Time::HiRes:: clutter.