use constant DEBUG => !!( 0 || $ENV{PERL_DEBUG_MYAPPNAME} );
use if DEBUG , qw' Carp::Always ';

use Perl::OSType qw' is_os_type ';
use if is_os_type('Windows'), qw' Win32::SerialPort ';
use if not is_os_type('Windows'), qw' Device::SerialPort ';

use if eval { require mod; 1 }, qw' constant haveMod 1 ';
use if not exist $INC{'mod.pm'}, qw' constant haveMod 0 ';
use if not exist $INC{'MIDI/ALSA.pm'}, 'something::else', ...;

perl -we " use if eval{require CGI}, our $cgi = 'CGI'; print $cgi->new->header "

perl -le " use if $ENV{maplat_debug}, lib => 666; print for @INC"