Did you mean to do this?

use strict;
use warnings;

my $string = join('x', ('a')x20);   ### NOTE the ()'s
print $string,$/;

my (@foo, $foo);

(@foo[0..18], $foo) = split('x', $string);
print $foo[0],$/;
print "Success!\n" if $foo eq "a";
__END__
axaxaxaxaxaxaxaxaxaxaxaxaxaxaxaxaxaxaxa
a
Success!
[download]

I don't quite understand your  join('x', 'a'x20); otherwise. Why join a scalar?

cheers

SK