#!/bin/env perl
use strict;
use warnings;
use Net::SFTP::Foreign ();
my ( $host, $user, $timeout ) = ( '<host>', '<user>', 30 ); # typically from a subroutine call

my $sftp = Net::SFTP::Foreign->new(
    host => $host,
    user => $user,
    timeout => $timeout,
    more => [
        -i => "/some/path/to/$user/.ssh/id_dsa", # the path to your key file
        -o => 'PreferredAuthentications=publickey',
        '-v',
    ],
);