use strict;

# start by extracting all unique information from file
my %addresses = ();
while (<>)
{
    my ( $addr, @aliases ) = split( /\s+/, $_ );
    foreach my $alias ( @aliases )
    {
        $addresses{$addr}->{$alias} = 1;
    }
}

# write out unique information in requested format
my @masterarray = ();
foreach my $addr ( keys %addresses )
{
    my @aliases = ();
    foreach my $alias ( keys %{$addresses{$addr}} )
    {
        push( @aliases, $alias );
    }
    push( @masterarray, [ $addr, @aliases ] );
}

# print out array
foreach my $masterline ( @masterarray )
{
    print( join( ' ', @{$masterline} ) );
    print( "\n" );
}
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When supplied with a data file matching your input it outputs the following:

C:\temp>perl -w test.pl testdata.txt
IPADDRESS alias1 alias2 alias5
IPADDRESS2 alias3 alias4 alias5
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