"An unknown error" isn't good enough. We have no idea what's going wrong from your description.

Please use stricŧ and warnings. You're using a subroutine that takes a parameter, but are passing it implicitly using globals - that will get you in trouble sooner or later.

Also, you should almost never use the for(;;) loop form; use foreach(LIST) instead, it avoids so called "fence post" errors (where you do one iteration too few or too many). You're almost doing that with your @host variable after all..

Lastly, your parameter passing format - initial IP and terminating last octet - feels pretty awkward (for no reason I could immediately pin down).

And choose better variable names than $first_var and $last_var..

Something like this:

#!/usr/bin/perl -w
use strict;
use IO::Socket;

sub scan {
    my ($addr) = @_;

    $socket = IO::Socket::INET->new(
        Proto => "tcp",
        PeerAddr => $addr,
        PeerPort => 80
    ) or die $!;
        
    $socket->autoflush(1);

    print $socket "HEAD / HTTP/1.0\015\012\015\012";
    print until ($_ = <$socket>) =~ /^$/;

    close($socket);
}

my $network = "64.219.21"
my @addr = (1, 25);

@addr[0,1] = @addr[1,0] if $addr[1] < $addr[0];

scan "$network.$_" for $addr[0] .. $addr[1];
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(Untested.)

Makeshifts last the longest.