#/usr/bin/perl -T

use strict;
use warnings;

my( $index_of_host, %HOSTLIST );

OLOOP: while(<DATA>){

 s/^\s*// , s/\s*$//; # eradicate outer whitespace
 next OLOOP until m/^Host\s+(\w+)$/;
 # untaint $1
 $index_of_host = $1;

  ILOOP: while(defined $index_of_host && my $line = <DATA>){

    $line =~ s/^\s*//; # eradicate inner outer whitespace
    $line =~ s/\s*$//; 

    # assume an empty line between 'Host' lists in config file
    undef $index_of_host && last ILOOP unless $line; 

    if( $line =~ m/^hostname\s+(\w+)$/ ){
     # untaint $1;
     $hostname = $1 && next ILOOP;     
    }

    if( $line =~ m/^User\s+(\w+)$/ ){
     # untaint $1;
     push @{$HOSTLIST{$index_of_host}->{$hostname}}, $1;
    }
  } 
}

__DATA__
#hosts
Host host1 
    hostname host1.mydomain
    User user1

Host host2 
    hostname host2.mydomain
    User user1

##</code><code>##

foreach my $indhost( keys %HOSTLIST ){

  foreach my $hname( keys %$indhost ){

    foreach my $user ( @{ $hname } ){
  
      #host
      print $hname
      #user
      print $user    
    }
  }
}

##</code><code>##

# optimised config file

host1.mydomain 
user1
user2

#hostundef
#user1
#user2

host2.mydomain 
user1
user2