I started looking at this based on the data as presented; however, there appears to be a question mark over whether all that whitespace actually exists. Here's solutions for both scenarios.

Without extra whitespace: 

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my ($host, %seen);

while (<DATA>) {
    chomp;

    if (m{^hostname\d\.com}) {
        say $host = $_;
    }   
    else {
        say unless $seen{$host}{$_}++;
    }   
}   

__DATA__
hostname1.com
Gateway FAIL
Gateway FAIL
Gateway FAIL
hostname2.com
Gateway FAIL
Gateway FAIL
Gateway FAIL

[download]

Output:

$ pm_del_group_dup_nospace.pl
hostname1.com
Gateway FAIL
hostname2.com
Gateway FAIL

[download]

With whitespace (as written):

#!/usr/bin/env perl

use strict;
use warnings;

my ($host, %seen);
my $host_re = qr{ \A ( hostname \d+ [.] com ) \s* \z }msx;
my $fail_re = qr{ \A ( .*? ) \s*? \z }msx;

{
    local $/ = "";

    while (<DATA>) {
        if (m{$host_re}) {
            $host = $1;
            print;
        }
        else {
            print unless $seen{$host}{(m{$fail_re})[0]}++;
        }
    }
}

__DATA__
hostname1.com

Gateway FAIL

Gateway FAIL

Gateway FAIL

hostname2.com

Gateway FAIL

Gateway FAIL

Gateway FAIL

[download]

Output:

$ pm_del_group_dup.pl
hostname1.com

Gateway FAIL

hostname2.com

Gateway FAIL

[download]

-- Ken

In reply to Re: Delete Duplicate Entry in a text file by kcott
in thread Delete Duplicate Entry in a text file by astronogun

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.