If you expect your input to be 'dirty', then regular expressions might serve you better:

use strict;
use Data::Dumper;

my %hosts;
while (<DATA>) {
  chomp;
  next if /^#/;
  $hosts{$1} = $2
    if /^(\w+)\s(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})/;
  warn "Garbage at line $. of DATA: $_\n" unless $1 and $2;
}
print Dumper(%hosts);
1;
__DATA__
host1 1.13.10.116 #comment
host2 21.90.30.31 #blah blah blah
host3 56.87.1.10
host4 13.16.17
#The following hosts are on network B
host5 57.98.18.10 #yadda yadda yadda
host6 106.10.3.12
[download]

Splits would be more efficient if you expect the input to be clean, though.