I read some thread saying ReadKey can do it, but not sure how I can do it in this scenario.

You could use non-blocking reads:

#!/usr/bin/perl

use Term::ReadKey;

ReadMode 3; # echo off

my $msg = "Somebody, stop me..";

while(1){
  print "$msg\n";
  sleep(1);

  my $key = ReadKey(-1);   # -1 means non-blocking
  if (defined($key) and $key eq "s") {
    $msg = "somebody, start me";
  }
}
ReadMode 0;
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