From "Advanced Perl Programming":

$SIG{ALRM} = \&timed_out;

eval {
    alarm(10);    
    $buf = <>;
    alarm(0);
};

if ($@ =~ /BLAH/) {
    print "Timed out.";
}

sub timed_out {
    die "BLAH"
}
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update
this will wait ten seconds for someone to enter something. the alarm(0) is to reset the timer if something does get entered (so &timed_out won't get called).

if you want high resolution timing (like 2.35), ye olde dromedary suggests using syscall to call the system timer routines (itimer).

-- ar0n || Just Another Perl Joe