Why your regex matches the string can be explained like this:

"(?=hello)" matches "hello"
".*"        matches "hello Ga"
"[^G]"      matches "g"
"[^a]"      matches "s"
".*"        matches nothing
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Your regex properly skips matching [^G][^a] with "Ga" but will happily match stuff after that. You probably want to do something like this with a negative look ahead:

( "hello Gags" =~ /^(?=hello)(?!.*Ga/ )&& print ("Matched") || print (
+"Unmatched")
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Update
To answer your question about how debugging, I find that throwing in a few capturing parens, can often help you to see what is being matched. Here is is a simple example:

use strict;
( "hello Gags" =~ /^(?=hello)(.*)([^G][^a])(.*)$/ ) ?  matches() : pri
+nt ("Unmatched");

sub matches {
  no strict 'refs';
  for (1..10) {
    my $tmp = ${$_} or next;
    print "\$$_ = '$tmp'\n";
  }
  return 1;
}

__OUTPUT__
$1 = 'hello Ga'
$2 = 'gs'
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