use strict;
use warnings;

while (<DATA>)
{
    s/(@Filetyp\s*?:)/\1c/;
    print;
}

__DATA__
@header_start
@Titel         : init 
@Filename      : test.c
@Filetyp       :
@Version       : 0001
@Produkt       : xx
@delivery      : xx
@date          : 20160610
@header_end

##</code><code>##

 0:54 >perl 1895_SoPW.pl
Possible unintended interpolation of @Filetyp in string at 1895_SoPW.pl line 18.
\1 better written as $1 at 1895_SoPW.pl line 18.
Global symbol "@Filetyp" requires explicit package name (did you forget to declare "my @Filetyp"?) at 1895_SoPW.pl line 18.
Execution of 1895_SoPW.pl aborted due to compilation errors.

17:41 >

##</code><code>##

17:44 >perl 1895_SoPW.pl
@header_start
@Titel         : init
@Filename      : test.c
@Filetyp       : c
@Version       : 0001
@Produkt       : xx
@delivery      : xx
@date          : 20160610
@header_end

17:44 >