Here is what it looks like as a hash of arrays. It's not as scalable as a hash of hashes, since it needs to grep through the array of aliases each time, but on small files you shouldn't notice.

use strict;

# Hash of arrays
# USAGE: $masterhash{$ipaddress} = @aliases
my %masterhash;

my $input_file  = "test.in";
my $output_file = "test.out";

open INPUT_FILE, "<$input_file" or die "Could not open $input_file\n";

# Loop through each line in the input file
while(my $line = <INPUT_FILE>)
{
  # Split line on spaces
  my @line_parts = split(/\s/, $line);
  
  # Grab the IP address
  my $ipaddress = shift @line_parts;
  
  # Loop through the aliases
  foreach my $alias (@line_parts)
  {
    # Search through the array for that IP address to determine
    # whether or not the current alias is already there
    # 
    # The value in $masterhash{$ipaddress} is an array reference, so
    # we have to wrap it with a @{} to use it as an array 
    unless(grep {$_ eq $alias} @{$masterhash{$ipaddress}})
    {
      # If we get here, its a new alias for this IP address so add it
      push @{$masterhash{$ipaddress}}, $alias;
    }
  }
}

# Printing output...
open OUTPUT_FILE, ">$output_file" or die "Could not open $output_file\
+n";

# Print one line per IP address
foreach my $ip (sort keys %masterhash)
{
  # Print the IP, followed by all of its aliases joined by spaces.
  print OUTPUT_FILE ("$ip ", join(" ", sort @{$masterhash{$ip}}), "\n"
+);
}
[download]

test.in

IPADDRESS alias1 alias2 alias5
IPADDRESS2 alias3 alias4
IPADDRESS2 alias3 alias5
IPADDRESS2 alias3 alias5
[download]

test.out

IPADDRESS alias1 alias2 alias5
IPADDRESS2 alias3 alias4 alias5
[download]

Hope this helps!