From looking at the scp and the ssh man pages - you get it via your shell by typing man 1 scp there is the hint that the -f option does not ask for passwords (and also leaves no ssh window open). scp has the -o command line option that allows to deliver options to ssh. So you have to try that. If you don't have success with that, you could try either open scp for writing and try to write the password to scp, but I've never done this myself. The thing I'd try would be

  open SCP, "| scp -o -l $filename $user\@$host:$target";
  print SCP, $password;
  # now pray that scp does its magic ...
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A quick test with OpenSSH shows this to work, using

scp -B $file $user\@$host:$target
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The -B on OpenSSH is equiv to your -f

then, you can either set a null passphrase on your private key and trust OS level security to keep your key private, or use the authentication agent (ssh-agent) and enter your passphrase before invoking the Perl utility that uses SCP.

you may be able to use Expect.pm to do password based SCP, but it's probably way more trouble than it's worth (at least according to me).

Try this: Net::SCP

--cs

There are nights when the wolves are silent and only the moon howls. - George Carlin