use strict;
use Data::Dumper;

# Uh, well spotted, typo fixed with the extra +
# which would have no effect anyway. :-)
# ysth suggested that putting () in map shortcircuts
# empty lines. It worked. Thanks. :-)
# Wow, even better, dropped that testing bit.

#my %hostlist = map { /^(\w+)\s(.*)/?($1,$2):() } (<DATA>);
my %hostlist = map { /^(\w+)\s(.*)/ } (<DATA>);

print Dumper(\%hostlist);

__DATA__
host1 1.1.1.1
host2 1.2.3.5
[download]

use strict;
use Data::Dumper;

my %hostlist;
{
    local $/;
    %hostlist = <DATA> =~ /^(\w+)\s(.*)/gm;
}
print Dumper(\%hostlist);

__DATA__
host1 1.1.1.1
host2 1.2.3.5
[download]

use strict;
use Data::Dumper;

my %hosts;
while (<DATA>) {
  next if /^\s*(?:#|$)/;  # ignore comments and empty lines
  /^([^\s#]+)\s+(.*)/;    # capture ip address and names
  $hosts{$_} = $1 foreach split /\s+/, $2;
}

print Dumper(\%hosts);

__DATA__
# IP Masq gateway:
192.168.0.80    pedestrian

# Primary desktop:
192.168.0.82    raptor1

# Family PC upstairs:
192.168.0.84    trex tyrannosaur family

# Domain servers:
205.212.123.10  dns1 brutus
208.140.2.15    dns2
156.63.130.100  dns3 cherokee
[download]

$VAR1 = {
          'dns3' => '156.63.130.100',
          'brutus' => '205.212.123.10',
          'raptor1' => '192.168.0.82',
          'trex' => '192.168.0.84',
          'cherokee' => '156.63.130.100',
          'dns1' => '205.212.123.10',
          'tyrannosaur' => '192.168.0.84',
          'dns2' => '208.140.2.15',
          'family' => '192.168.0.84',
          'pedestrian' => '192.168.0.80'
        };
[download]