perlsub explains the semantics of the different calling syntaxes. One little-known difference is that &foo, with no parentheses, gives foo access to the current contents of @_:

#!/usr/local/bin/perl

sub foo {
  $_[0] = 'b';
}

@_ = 'a';

foo();
print "@_\n";

&foo;
print "@_\n";

__END__
a
b
[download]