I just have the following two line in my code and I toggle the comment depending on whether I'm testing or not:

MIME::Lite->send( smtp => 'real.smtp.server.net' ) ;
# MIME::Lite->send( smtp => 'localhost:10025' ) ;
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Although that might just confirm: "There's more than one way to do it" doesn't always mean there's more than one way you should do it.

I'm not sure what the "system call" has do do with your issue (you don't show any sample code), but maybe the following will give you a different tool to play with:

#!/usr/bin/perl
use strict;
use warnings;
use Test::MockObject;

BEGIN {
    my $mockMIMELite = Test::MockObject->new();
    my %params = (To => 'parp', Subject => 'Beep', From => 'Bop');
    $mockMIMELite->fake_module(
        'MIME::Lite',
        new  => sub {ML_new($mockMIMELite, @_)},
        send => sub { },
        );
    $mockMIMELite->mock(send   => \&ML_send);
    $mockMIMELite->mock(attach => \&ML_attach);
}


use MIME::Parser;

my $msg = MIME::Lite->new(
                To      => 'wibble@wobble',
                From    => 'Bogus email source',
                Subject => 'Email test',
                Data    => 'Email body',
                );
$msg->send();


sub ML_new {
    my ($mock, $class, %params) = @_;

    $mock->{newParams} = \%params;
    return $mock;
}


sub ML_send {
    my ($self)     = @_;
    my %fields     = %{$self->{newParams}};
    my @headerKeys = (
        (grep {exists $fields{$_}} qw(To From Cc Subject)),
        (grep {!/^(data|subject|cc|to|from)$/i} sort keys %fields)
        );

    print "--------------8<------------------\n";
    print "$_: $self->{newParams}{$_}\n"
        for grep {defined $self->{newParams}{$_}} @headerKeys;
    print "Data:\n$self->{newParams}{Data}\n";
    print "-------------->8------------------\n\n";

    return 1;
}


sub ML_attach {
    my ($self, %params) = @_;
    my %fields = %{$self->{newParams}};

    print ".............-8<..................\n";
    print "Type:        $params{Type}\n";
    print "Path:        $params{Path}\n";
    print "Filename:    $params{Filename}\n";
    print "Disposition: $params{Disposition}\n";
    print ".............-8<..................\n";

    return 1;
}
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Prints:

--------------8<------------------
To: wibble@wobble
From: Bogus email source
Subject: Email test
Data:
Email body
-------------->8------------------
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$ perl -le " print CGI->version "
Can't locate object method "version" via package "CGI" (perhaps you fo
+rgot to load "CGI"?) at -e line 1.

$ set PERL5OPT=-MCGI

$ perl -le " print CGI->version "
3.59
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thisname: /goes/to/file
thatname: "| goes to a program"
andthat: "|/bin/logger -p user.notice -t testmail" # goes to syslog
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