use Sub::Prepend 'prepend';

sub foo ($) {
  print "Foo executed with \@_ = (@_).\n";
}

BEGIN {
  prepend foo => sub {
    # This is called before foo executes.
    print "Foo was called with \@_ = (@_).\n";
  };
}

my @bar = qw/ foo bar baz /;
foo(@bar); # The prototype is preserved!

__END__
Foo was called with @_ = (3).
Foo executed with @_ = (3).

##</code><code>##

my $old = \&foo;
*foo = sub { ...; goto &$old };

##</code><code>##

use Scalar::Util 'set_prototype';

my $name = ...;

my $old;
my $new = sub { ...; goto &$old };
{
  no strict 'refs';
  $old = \&$name;
  set_prototype(\&$new, prototype $old);
  no warnings 'redefine';
  *$name = $new;
}

##</code><code>##

prepend(foo => sub { ... });

##</code><code>##

prepend(foo => sub ($) { ... });
prepend(foo => sub { ... }, { prototype => '$' });