wendy24,
You are still omitting details that would allow us to provide a full working solution such as the width of each field. Additionally, do the records have a newline separator or is it just one long runone line?

#!/usr/bin/perl
use strict;
use warnings;
my ($in, $out) = @ARGV;
die "Usage: $0 <input file> <output file>" if ! defined $in || ! defin
+ed $out;
open(my $in_fh, '<', $in) or die "Unable to open '$in' for reading: $!
+";
open(my $out_fh, '>', $out) or die "Unable to open '$out' for writing:
+ $!";

while ( <$in_fh> ) {
    chomp;
    my ($name, $add, $city, $state, $zip) = unpack('A10A20A15A12A5', $
+_);
    $zip = 15206 if uc($city) eq 'PITTSBURGH';
    print $out_fh $name, $add, $city, $state, $zip, "\n";
}
[download]

Of course, this assumes that name is only 10 characters long and zip is 5 but they can be adjusted accordingly. It also assumes the records are newline separated and will not work otherwise. Don't worry about being new but think about what information is needed to solve the problem even if you don't know how to solve it yourself.