You probably want to use a hash instead. Create a key for each zipcode by assigning it the value of 1, then see if the zipcode you're checking has a key in the hash.

#!/usr/bin/perl
use strict;
use warnings;

my $file = "zipcodes.txt";
my %zipcodes;

open (my $fh, '<', $file) or die "Can't open $file for read: $!";

while (<$fh>) {
  chomp;
  $zipcodes{$_}++;
}

close $fh or die "Cannot close $file: $!";

while (chomp( my $line = <STDIN> )) {
  print "found $line\n" if $zipcodes{ $line };
}
__END__
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Don't forget to chomp the lines as you read them from the file, and from STDIN.

Edit: If you want to be lazy, you can use a module to open, read and close your file, do the chomping for you and put the lines in a list. In the snippet below we use Path::Tiny, which allows us to write one line to return a list of zip codes and map them into the hash:

use Path::Tiny;
my $file = 'index.txt';
my %zipcodes = map { $_, 1 } path( $file )->lines({chomp => 1});
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More simply, this is how to get the list of chomped lines:

my @lines = path( $file )->lines({chomp => 1});
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Hope this helps!