$ perl -le'
my @fields = (
  [ name1 => { start =>  0, len=> 10 } ], 
  [ name2 => { start =>  8, len=>  3 } ],
  [ name3 => { start => 11, len=> 14 } ],
);

my $format = join q[ ], map sprintf( q[x%d a%d X%d], @{ $_->[ 1 ] }{ qw[start len] }, $_->[ 1 ]{ start } + $_->[ 1 ]{ len } ), @fields;
                                                                    
print $format;
'
x0 a10 X10 x8 a3 X11 x11 a14 X25