If your lines are simply host and IP separated by a space, then it's pretty simple:

open my $fh, "<input.dat" or die "Couldn't open file: $!";

my %hosts;
while (<$fh>) {
    chomp;
    my ($host, $ip) = split;
    $hosts{$host} = $ip;
}

use Data::Dumper;
print Dumper \%hosts;
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