Hi I have got another explanation that Using & means that it's a user defined subroutine. For example:
&print is a user define print and will not use Perls print function.
Also, if you don't specify the "&", subroutines must be defined before you make a call to it.
But from PErl Best Practise:

sub lock{
   my ($file)=@_;
   print " user defined lock\n";
  
}
sub link{
  my ($text, $url) =@_;
  print " user defined link $text $url\n";
}
print "calling lock: ".lock;  #calls locl, built-in lock hidden;
print "calling &lock: ".&lock; # calls user defined lock
print "calling &link: ".&link("123","123"); # user defined link
print "calling &link: ".&link; #user defined won't print 123
print "calling link: ".link("123","123");  # built-in link
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So my question: is this a strong case for using &?