The good:

• 3 argument open. Very much safer than the two arg flavor.
• automating the parameter collection

The bad:

• input parsing is overly complicated.
• Big chains of and/or logic are hard to maintian.
• <> has extra magic. I'd just use STDIN. Unless you want the magic.
• No quotes needed in hash keys.
• High precedence logical or (||) in the line where you open the input file doesn't do what you think it does.

use strict;
use warnings;

my %settings; 
foreach my $key (qw(input output pattern replacement)) {
    print "$key = "; 
    my $setting = <STDIN>;
    chomp $setting;
    $settings{$key}=$setting;
}

open IN, '<', $settings{input} 
    or die "Can't open input file $settings{input}: $!"; 

die "Output file $settings{output} already exists\n" 
    if  -e $settings{output};

open OUT, '>', $settings{output} 
    or die "Can't open output file $settings{output}: $!";

while (<IN>) {
    s/$settings{pattern}/$settings{replacement}/g;    
    print OUT $_;
}

close (IN);
close (OUT);
[download]

Now, there are still some problems with this code. The big one is that using a hash and keys for storing the settings, looses us the benefits of using strictures in the first place. If I misspell 'output' when I use it, I'll get no warnings. Perl happily autovivifies a value in the hash and returns an undef.

There are ways around this issue. The easiest is to use Hash::Util to lock the hash, so that attempts to change the hash or access nonexistent keys become a fatal error.

use Hash::Util qw(lock_hash);
my %settings; 
foreach my $key (qw(input output pattern replacement)) {
    print "$key = "; 
    my $setting = <STDIN>;
    chomp $setting;
    $settings{$key}=$setting;
}
lock_hash %settings;
[download]

Another thing is the use of global file handles. I should really be using lexical filehandles. Global handles are global variables, with all the attendant problems.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.