#!/usr/bin/perl
use strict;

die "Usage: $0 startdir search replace\n"
    unless scalar @ARGV == 3;
my $startdir = shift @ARGV || '.';
my $search = shift @ARGV or
    die "Search parameter cannot be empty.\n";
my $replace = shift @ARGV;
...

|| '.'

or die "Search parameter cannot be empty.\n";