Keep in mind that each line in the hosts file can define multiple aliases:

#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dump qw/dump/;
my $hostfile = "/etc/hosts";
my %hosts = ();
open FILE, "<", "$hostfile" || die "Cannot open $hostfile $!";
while (<FILE>) {
  next if /^\s*#/ ; # skip comments
  chomp;
  my ($key, $values) = split /\s+/, $_, 2;
  my @values = split /\s+/, $values;
  $hosts{$key} = \@values;
}
print dump(\%hosts), $/;
__END__
{
  "127.0.0.1" => ["localhost.localdomain", "localhost"],
  "127.2.3.4" => ["app.example.com"],
  "127.2.3.5" => ["www.example.com", "www", "web", "catalyst"],
  "127.2.3.6" => ["cisco.example.com", "router"],
  "127.2.3.7" => ["mail", "mail.example.com", "mx1"],
}
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Thank you for your reply. I changed my code to the following.

#!/usr/bin/perl -w

use strict;

my $hostfile = "/etc/hosts";
my %hosts = ();


open FILE, "<", "$hostfile" || die "Cannot open $hostfile $!";
while (<FILE>) {
    next if /^\s*#/ ; # skip comments
    chomp;
    my ($key, $value) = split (" ", $_);
    $hosts{$key} = $value;
    }
close FILE;
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The output is as follows.

Use of uninitialized value $key in hash element at hash.pl line 14, <F
+ILE> line 3.
Use of uninitialized value $key in hash element at hash.pl line 14, <F
+ILE> line 11.
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Why do I get the "uninitialized error" for $key?

Why do I get the "uninitialized error" for $key?

My guess is your input file has a blank line:

while (<FILE>) {
    next if /^\s*#/ ; # skip comments
    next unless /\S/; # skip blank lines
    chomp;
    my ($key, $value) = split;
    $hosts{$key} = $value;
    }
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See perlre

Your /etc/hosts file lines 3 and 11 have zero or more white space characters and nothing else. You should skip those:

while (<FILE>) {
  next if /\A\s*\z/ ; # skip blank lines
  next if /^\s*#/ ; # skip comments
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Or combine the regexes:

  next if /\A\s*(?:#|\z)/ ; skip comments and blank lines
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