Your mileage may vary but here's how I do it with Net::SFTP::Foreign:

#!/bin/env perl
use strict;
use warnings;
use Net::SFTP::Foreign ();
my ( $host, $user, $timeout ) = ( '<host>', '<user>', 30 ); # typicall
+y 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 ke
+y file
        -o => 'PreferredAuthentications=publickey',
        '-v',
    ],
);
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