Subroutines don't "send", they "return".
When you say "input", you seem to mean "argument".
Please learn the terminology. It'll help make your posts understandable.

in &testmode the &makeitbig is not returning anything is expecting a value to process

No, not nothing. It's returning the value in $default_value. Did you forget to assign something meaningful to $default_value?

is it possible or how to send the return value of a subroutine A as an input arguments to subroutine B ?

Sure, by placing a subroutine call inside the parens which delimit another sub-call's arguments. If

sub A {
    return $_[0] x 2 . ' (from A)';
}
sub B {
    my $arg = shift;
    return "B got $arg as argument\n";
}
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then

print B(A("c")),"\n";
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results in

B got cc (from A) as argument
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The return value from A() ends up at the right place - inside the parens of B().

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

sub makeitbig {
  my $default_value = 'xxx';
  if (@_) {
    $default_value = shift;
  }
  return $default_value;
}
sub testmode {
  my $random1 = int( rand(makeitbig()) );
  return $random1;    
}
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# not tested, but you get the gist
sub makeitbig {
  return @_ ? shift @_ : 'xxx';
}
sub testmode {
  return int( rand(makeitbig()) );    
}
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