Examples of mentioned solutions:

• $data = $data . $more;
• $data .= $more;
• substr($data, length($data), 0, $more);
• substr($data, length($data)) = $more;
• $data = join('', $data, $more);
• $data =~ s/\z/$more/;
• $data =~ "$data$more";
• { open(my $fh, '>+', \$data); print $fh $more; } (Requires 5.8)

Other useful solutions:

• $data = sprintf('%s%s', $data, $more);
• $data = pack('a*a*', $data, $more);
• $data = pack('(a*)*', $data, $more); (Requires 5.8)
• $data = do { local $"; my @array = ($data, $more); "@array" };

Other text formatting functions:

• format formats text sent to a file handle.
• formline is the guts of format.

And that's only using tools meant for text manipulation and/or formatting. You could do weird stuff like:

• $data = reverse scalar reverse $data, $more;
• $data = do { local $;; my %hash; $hash{$data, $more}++; (keys(%hash))[0] };