#!/usr/bin/perl

# https://perlmonks.org/?node_id=11104382

use strict;
use warnings;
use IO::Select;
use Time::HiRes qw(time);
use List::Util qw( max );

sub Engine::doHeatbeat
  {
  print "in Engine::doHeatbeat\n";
  }

sub Engine::Parser::Command
  {
  print "in Engine::Parser::Command( @_ )\n";
  }

my $interval = 10; # seconds
my $sel = IO::Select->new(*STDIN);
my $trigger = time + $interval; # unix time to call heartbeat

while(1)
  {
  my $wait = max 0, $trigger - time;
  for my $fh ( $sel->can_read( $wait ) )
    {
    chomp( my $input = <$fh> );
    Engine::Parser::Command($input);
    }
  if( $trigger <= time )
    {
    Engine::doHeatbeat;
    $trigger = time + $interval;
    }
  }
[download]

Consider this an example of why you should use an Async package as recommended by Hippo here Re: Infinite LOOP and reading from STDIN