Net::SSH::Perl package, in Channel.pm, line 142,

142:                $r->{code}->( $c, $c->{$buf}, @{ $r->{extra} } );

in sub process_buffers,

sub process_buffers {
    my $c = shift;
    my($rready, $wready) = @_;

    my %fd = (output => $c->{wfd}, extended => $c->{efd});
    for my $buf (keys %fd) {
        if ($fd{$buf} && grep { $fd{$buf} == $_ } @$wready) {
            if (my $r = $c->{handlers}{"_${buf}_buffer"}) {
                $r->{code}->( $c, $c->{$buf}, @{ $r->{extra} } );
            }
            else {
                #warn "No handler for '$buf' buffer set up";
            }
            $c->{local_consumed} += $c->{$buf}->length
                if $buf eq "output";
            $c->{$buf}->empty;
        }
    }

    if ($c->{rfd} && grep { $c->{rfd} == $_ } @$rready) {
        my $buf;
        sysread $c->{rfd}, $buf, 8192;
        ($buf) = $buf =~ /(.*)/s;
        $c->send_data($buf);
    }

e.g.

#!/usr/bin/perl -w

use strict;

my $hash_ref;

$hash_ref->{add} = sub{
        my $ten = shift;
        my $twenty = shift;
        my $numbers = shift;
        my $sum;

        $sum += $_ for @{$numbers};

        print "$ten $twenty $sum\n";