#! /usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @list = qw( Liste mit 1000 Elementen und eins mehr);

my $separator = '';    # empty string

my @new;
while ( my @join_these = splice( @list, 0, 2 ) ) {
    no warnings 'uninitialized';
    push @new, join( $separator, @join_these );
}

print Dumper \@new;

__END__