#! perl -sw
use strict;
use Term::ReadKey;
use Time::HiRes qw(gettimeofday tv_interval usleep);

my $start = [gettimeofday];

usleep 10_000, 
      printf "\r%6.2f Press Enter when ready", tv_interval $start
	while !defined ReadKey(-1);

print $/, 'You took ', tv_interval($start), ' seconds to hit the Enter key', $/;

__DATA__
# Ouput
C:\test>203582
  7.33 Press Enter when ready
You took 7.331 seconds to hit the Enter key

C:\test>