use DateTime::Format::Epoch::MJD qw( );

print( 
   DateTime::Format::Epoch::MJD->parse_datetime( $mjd )->ymd(),
   "\n"
);
[download]

But contrary to what you said 39749 is 1967-09-16. 2008-10-28 is 54767.

Update:

You can make your own DateTime::Format::Epoch derivative.

BEGIN {
   package DateTime::Format::Epoch::Excel;

   use DateTime                qw( );
   use DateTime::Format::Epoch qw( );

   our @ISA = 'DateTime::Format::Epoch';

   my $epoch = DateTime
               ->new( year => 1900, month => 1, day => 1 )
               ->subtract( days => 2 );

   # Incorrect for days before March 1st, 1900 
   # due to 1900 being treated as a leap year.

   sub new {
      my ($class) = @_;
      return $class->SUPER::new(
         epoch => $epoch,
         unit  => 1/86400,
         type  => 'float',
         skip_leap_seconds => 1
      );
   }
}

print(
   DateTime::Format::Epoch::Excel->parse_datetime( 39749 )->ymd(),
   "\n"
);
[download]

You can use DateTime::Format::Excel.

use DateTime::Format::Excel qw( );

print(
   DateTime::Format::Excel->parse_datetime( 39749 )->ymd(),
   "\n"
);
[download]