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);