c:\@Work\Perl\monks\iaw4>perl -wMstrict -le
"use 5.010;
 ;;
 my $s =
   'ab + (fun1( fun2(  3  +fun3 ( 4+(5+6)    ))) ) + x + f  ( 1 + 2 )';
 print qq{'$s'};
 ;;
 $s =~ s{ [(] (?: [^()]++ | (?0) )* [)] }
        { (my $ns = ${^MATCH}) =~ s/\s+//g;  $ns; }xmsgpe;
 print qq{'$s'};
"
'ab + (fun1( fun2(  3  +fun3 ( 4+(5+6)    ))) ) + x + f  ( 1 + 2 )'
'ab + (fun1(fun2(3+fun3(4+(5+6))))) + x + f  (1+2)'