Something like this?

$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)
[download]

I put numbers in so I could see what was going on, but it works fine with spaces too.

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

a( )( )
a(  )( )
a(   )( )
a(    )( )
a(     )( )
a(      )( )
[download]

Update: Or if you like cute, you can do away with the external loop and use backtracking.

$s ='a       ';
$s =~ m[^a(.+?)(.)(?{ print "a($1)($2)" })(?!)];

a( )( )
a(  )( )
a(   )( )
a(    )( )
a(     )( )
a(      )( )
[download]