Your code exits the sub after the 1st line of your file is read because the return is inside the while loop. This prints each host within the sub. Nothing is returned explicitly:

use warnings;
use strict;

sub get_host {
    open( HOST, "<hosts.txt" );
    while (<HOST>) {
        my ($line) = $_;
        chomp($line);
        print "hostname = $line\n";
    }
}
get_host();

__END__

hostname = hostone
hostname = hosttwo
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