You can use the select function like this for non-blocking input. This isn't exactly what you asked for because it won't respond to a simple keypress, you have to enter a string ending with a newline (Enter key).

YuckFoo

#!/usr/bin/perl

   use strict;

   my $timeout = 5;

   while (1) {
      my $str = getstr($timeout);
      if ($str ne '') { print "Got it! $str"; }
      else            { print "I got nothing.\n"; }
   }

#-----------------------------------------------------------
sub getstr {

   my ($timeout) = @_;
   my ($rin, $rout);

   vec($rin,fileno(STDIN),1) = 1;

   print "You have $timeout seconds to enter a string:\n";

   my $input = select($rout=$rin, undef, undef, $timeout);

   if ($input) { return <STDIN>; }
   else        { return ''; }
}
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