my $VALUE  = do {
    use re 'eval';
    qr/$NUM(?{ print "($`)($&)($')\n" })(?!)/;
};

my $text = 'name => 5, fav.num => 3';
$text =~ /$VALUE/;
__END__
(name => )(5)(, fav.num => 3)
(name => )()(5, fav.num => 3)
(name => 5, fav)(.)(num => 3)
(name => 5, fav)()(.num => 3)
(name => 5, fav.num => )(3)()
(name => 5, fav.num => )()(3)