For this case, where you only want one item, reverse your split:

use strict;
use warnings;

my $string = join('x', ('a') x 20);

my (@foo, $foo);

($foo, @foo) = reverse( split('x', $string) );

print "\$foo:$foo\n";
print "\@foo:@foo\n";
[download]

Prints

$foo:a
@foo:a a a a a a a a a a a a a a a a a a a
[download]

Note the fix in your $string line to put 'a' as a list