If you know two slightly less well known things about Perl then you can:

#!/usr/bin/perl
use strict;
use warnings;

open my $hostsIn, '<', "/etc/hosts" or die "Cannot open hosts file! \n
+";
my @hosts = <$hostsIn>;
close $hostsIn;

foreach my $line (@hosts) {
    next if $line !~ /($ARGV[0])/;

    print $line;
    $line = undef;
}
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For that to make sense you need to know that delete sets an array element to undef (that is a lie, but it suffices at present - see the linked docs) and that the loop variable is an alias to the list elements being iterated over by a for loop.

Note too the use of strictures (use strict; use warnings; - see The strictures, according to Seuss), lexical file handles and the three parameter open. Getting into the habit of using these techniques will save you a lot of time and frustration!