#!/usr/bin/perl 
use strict; 
use warnings FATAL => qw( all ); 
 
#$/ = "\r\n"; 
$" = ", "; 
 
my $filepath = 'input.txt'; 
my $key      = undef; 
my $hash     = { }; 
 
# parse 
open my $filehandle, '<', $filepath || die "cannot open $filepath: $!"; 
while ( <$filehandle> ) 
{ 
    chomp; 
 
    if ( /^\s*(application|region|server)\s*$/i ) 
    { 
        $key = lc $1; 
    } 
    elsif ( defined $key && /^\s*(.+)\s*$/ ) 
    { 
        push @{$hash->{$key}}, $1; 
    } 
} 
close $filehandle; 
 
# print 
while ( my ( $key, $vals ) = each( %$hash ) ) 
{ 
    print "$key, @$vals\n"; 
}

region, NA  
application, ABC

use warnings; 
use strict; 
 
my %data; 
@data{qw(Application Region Server)} = undef; 
my $key; 
 
while (<DATA>) { 
    chomp; 
    next if /^$/; 
    if ( exists $data{$_} ) { 
        $key = $_; 
    } 
    else { 
        push @{ $data{$key} }, $_ if defined $key; 
    } 
} 
 
while ( my ( $key, $value ) = each %data ) { 
    print $key, ' ', join( ", " => @$value ), $/; 
} 
 
__DATA__ 
 
 
Details 
 
 
Application 
 
ABC 
 
Region 
 
 
 
NA 
 
Server 
 
 
 
Atom.prod 
 
Application 
 
 
 
NA  
 
 
Region 
 
 
XYZ 
 
Server 
 
 
 
Atom.prod2

Server Atom.prod, Atom.prod2 
Application ABC, NA  
Region NA, XYZ