I don't have any trouble using a DSA key but for the life of me cannot get it to use an RSA key as you have done.

Here is my working (but scrubbed) DSA key script.

#!/bin/env perl
use strict;
use warnings;
use Net::SSH::Perl;

my @ids = ('/path/to/id_dsa');
my %params = (
        protocol        => 2,
        interactive     => 1,
        identity_files  => \@ids,
        debug           => 1,
);
my $host = 'somehostwithssh';
my $ssh = Net::SSH::Perl->new( $host, %params ) or die "Could not crea
+te SSH object for $host\n";
$ssh->login('username');
my ( $out, $err, $exit ) = $ssh->cmd( 'ls /tmp' );
print "out: $out\n" if defined $out;
print "err: $err\n" if defined $err;
print "exit: $exit\n" if defined $exit;
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