If you are prepared to replace the string "Hello ".$a; with a function which constructs the string on-the-fly then:

my $x;
my $str = sub { "hello ".${\$x} };
$x = 'kevin';
print $str->();
$x = 'chickenman';
print $str->();
[download]

This resembles hippo's Re: Evaluate variable while using (which is simpler as it achieves the same without the use of refs).

A more complete and acceptable solution would be to use an OO approach where a class holds the data and offers methods to update and print the data and any subsequent strings you may want to produce, i.e. $executable. This is a textbook usage for OO which keeps its data and its methods in their castle with crocodiles all around.

{package XYZ;
sub new {
  my $class = shift;
  my $self = {
   homedir => '<empty>'
  };
  return bless $self => $class
}
sub homedir {
  my $self = $_[0];
  if( defined $_[1] ){ $self->{homedir} = $_[1] }
  return $self->{homedir}
}
sub executable { my $self = $_[0]; return $self->homedir()."/xyy/abc" 
+}
1;
}
my $xyz = new XYZ();
$xyz->homedir("ahah");
print "homedir: ".$xyz->homedir()."\n";
print "exec: ".$xyz->executable()."\n";
# note, when dealing with paths use File::Spec->catdir()
[download]

That's probably a good approach.

bw, bliako