=> is a glorified comma. So now we actually have:

grep /regex/, @_;
#    EXPR   , LIST
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The regex is an expression. @_ can provide data like a list. And thus it is grep EXPR, LIST.

The "fat arrow" => is often also called the "fat comma", because it functions the same as the comma. So the following two pieces of code are the same, functionally:

  grep /regex/ => @_;
  
  # and

  grep /regex/, @_;
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I think that perldoc perlop discusses the peculiarities of the fat arrow, but for many cases, you can simply replace the arrow by a comma.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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Ouch! I knew => was a comma in hash definition from the Llama but I didn't make the connection, sorry. But why would an author be using this form

unless (grep /regexp/ => @_) {...}
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? Is it just easier to read than it would be using a simple comma? Or does the special property of interpreting the word to the left of => as a string have a beneficial effect? Thank you for your kind help.

I think the fat comma is used here for aestethical reasons, as /regexp/ is applied to @_. I am also guilty of that sometimes. The notation makes sense sometimes, originally for key-value pairs in hashes :

  my %hash = ( foo => bar );
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You can also use it for the parameters if you have a subroutine that acts as a pipeline :

  sub transform {
    my ($source, $target) = @_;
    ...
  };

  transform( 'input/start.file' => 'output/' );
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In that case, the "arrow" could be interpreted also in the mapping sense.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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%hash = (this => 'that', now => 'then');
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%hash = ('this', 'that', 'now', 'then');
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