1.) ($concat = $a) .= $b;

##</code><code>##

2.) $" = $b; @_ = ($a,undef); $concat = "@_";
3.) do { $concat .= $_ for (split(//))} for ($a,$b);
4.) my %a = ($a => $b); $concat = join'',each %a;
5.) $concat =~ s//$a/; $concat =~ s/$/$b/;
6.) $concat = <<"EOF"; chomp($concat);
    $a$b
    EOF
7.) $concat = `echo -n "$a$b"`;  # for safe values of $a and $b
8.) $concat = glob("{$a}{$b}");  # not Taint safe... will choke on metachars...
9.) ($concat) = ("$a$b" =~ /(.*)/);

And finally, everyones favorite Perl6 construct...
10.) $concat = $a _ $b;