#!/usr/bin/perl -w 
 
use strict; 
 
my $data = join '', <DATA>; 

$data =~ s|(?=^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\nOS Type: unknown$)|
+\n|gm; 
 
print $data; 
 
exit(1); 
 
__DATA__
10.1.1.1
bogus info
10.1.1.2
this could be anything
10.1.1.3
OS Type: unknown
10.1.1.4
filler information
10.1.1.5
OS Type: unknown
[download]

Although the right solution is to use multiline mode, your code doesn't do anything as the test on the regex reveals. See here:

#!/usr/bin/perl -w 
 
use strict; 
 
my $data = join '', <DATA>; 

if($data =~ s|(?=^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\nOS Type: unknown
+$)|\n|gm)
{
    print "yes, it worked!";
} 
 
#print $data; 
 
exit(1); 
 
__DATA__ 
10.1.1.1 
bogus info 
10.1.1.2 
this could be anything 
10.1.1.3 
OS Type: unknown 
10.1.1.4 
filler information 
10.1.1.5 
OS Type: unknown
[download]

The above code returns nothing, meaning that the regex never does the substitution. From a quick rereading of the algorithm, it looks like he is trying to do the following:
1. Read through the file
2. find lines that start with IP addresses
3. ASK if the next line following it has some specific text
4. write an extra space BETWEEN the IP address line and the specific text line
I may be wrong, as the spec. was pretty poorly written. A quick command line example that does this is as follows:

perl -e "$q = qq(the brown dog\n is hailing a cab); if (($q=~m/\n(.*)/
+mg) && ($1 eq q( is hailing a cab))) { print qq(\n$1;)} else {print '
+no luck buddy.';}"
[download]

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Actually, the regex "mostly works".

The problem with this regex is that the DATA does not have an immediate newline following it (there is a space). If the space is removed - or a simple (.+)? is added between the IP match and the newline, the first regex works fine. I'll leave someone else to do the benchmarks :-)

That's of course, if the original poster wants the new line before the line with the IP