You could try something like this old code I had lying around (translation: this is old and I don't know if it still works with recent versions of LWP). Set $ENV{LWP_LOCAL_ADDR} before you create each instance and it'll use that for the local socket address.

package MyHTTP; 
use base qw(LWP::Protocol::http); 

sub _new_socket { 
    my($self, $host, $port, $timeout) = @_;

    local($^W) = 0;  # IO::Socket::INET can be noisy
    my @args = (PeerAddr => $host,
                PeerPort => $port,
                Proto    => 'tcp',
                Timeout  => $timeout,
               );
    push @args, LocalAddr => $ENV{LWP_LOCAL_ADDR}
        if exists $ENV{LWP_LOCAL_ADDR};

    my $sock = IO::Socket::INET->new( @args );

    unless ($sock) {
        # IO::Socket::INET leaves additional error messages in $@
        $@ =~ s/^.*?: //;
        die "Can't connect to $host:$port ($@)";
    }
    $sock;
}

LWP::Protocol::implementor( 'http', 'MyHTTP' );

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You also could alter it from using %ENV to maybe look at a global instead. I'm not aware of a better way of passing options down through from the LWP::UserAgent level (not to mention from Mechanize) to this level though; would be nice if you got a reference to that instance as an arg to _new_socket.

_extra_sock_opts should overriden instead of _new_socket for forward compatibility. For example, your solution breaks connection caching.

package MyHTTP; 
use base qw(LWP::Protocol::http); 

sub _extra_sock_opts {
   my ($self, $host, $post) = @_
   my @opts = $self->SUPER::_extra_sock_opts($host, $post);

   push @opts, LocalAddr => $ENV{LWP_LOCAL_ADDR}
      if exists $ENV{LWP_LOCAL_ADDR};

   return @opts;
}

LWP::Protocol::implementor( 'http', 'MyHTTP' );

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or just

push @LWP::Protocol::http::EXTRA_SOCK_OPTS,
     LocalAddr => $ENV{LWP_LOCAL_ADDR}
   if exists $ENV{LWP_LOCAL_ADDR};
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