... an alarm routine will trigger and exit the script.

You answered your own question :)

an alarm routine will trigger and exit the script

besides alarm...

perldoc -f select

Try this snippet( from a lost node somewhere :-) ):

#!/usr/bin/perl -w
use strict;

my $input = &my_stdin(
   prompt =>"Enter some text: ",
   timeout => 3,
   handler => \&my_alarm );


print "You entered: $input \n";

sub my_stdin {

   use Term::ReadKey;
   $| = 1;     #turn buffering off so we can see our print below
   my %args    = @_;
   my ($output,$key );
   my $prompt  = $args{'prompt'};
   my $timeout = $args{'timeout'};
   my $toh     = $args{'handler'};  # ref to a handler

   $SIG{ALRM} = $toh || sub { print "Timeout\n"; exit;};
   alarm($timeout);
   print $prompt;
   ReadMode 4;
   while(1){
       if(defined($key = ReadKey(-1))){
       my $val = ord $key;
       # 8 is backspace 10 and 13 are
       $output .= $key unless $val == 8 || $val == 10 || $val == 13;
       #echo to the screen
       print $key if length $output > 0;          
       alarm($timeout);     #reset the alarm
           #last if the user hits return
       last if ( $val == 10 || $val == 13);
       # chop one if we get a backspace
       if( $val == 8 ){ chop $output; print " "; print chr(8); }
       }
   }
   ReadMode 0;
   print "\n";
   $output;
}

sub my_alarm{
   print "I just died because you waited too long!\n"; exit;        
}

If you want to use an event loop system, like Tk, Gtk2, or POE, you can do a neater job. Tk or Gtk2 make it trivial, but you need to be running under X or Windows. You just setup an event to be fired after the timeout you give.