#! /usr/local/bin/perl -w

use strict;

my $re = shift || '.';
$re    = qr/$re/;

while( <DATA> ) {
    print if /$re/;
}

__DATA__
Owing to changes in recent perls (5.8+ I believe), signals no longer
interrupt a single opcode's execution. A regex is a single opcode,
so the alarm never interrupts it. One solution, as mentioned above,
is to use unsafe signals, although I am unsure if it is merely an
ENV variable or a compile option. As the name says, these are
potentially unsafe as a signal may interrupt an opcode that isn't
interruptible and thus crash perl, but this is a very rare case.