Batch rename files with Perl reg

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

# Larry Wall's filename fixer: recipe 9.9 in Perl Cookbook 

$op = shift 
	or die "Usage: $0 expr [files]\n";
chomp(@ARGV = <STDIN>) unless @ARGV;
for (@ARGV) {
    $was = $_;
    eval $op;
    die $@ if $@;
    rename($was, $_) unless $was eq $_;
}