$s ='a123456789';
$p = 0; 
print "a($1)($2)\n" while ++$p and $s =~ m[^a(.{$p})(.)];

a(1)(2)
a(12)(3)
a(123)(4)
a(1234)(5)
a(12345)(6)
a(123456)(7)
a(1234567)(8)
a(12345678)(9)