use Digest::MD5 qw(md5);

my $message = ...blah blah blah...;

my $md5_test = my_md5($message);
my $md5_chek = md5($message);

print "MESSAGE: '$message'\n"
.     "TEST:    '$md5_test'\n"
.     "CHECK:   '$md5_chek'\n";
print "*** MISMATCHED ***\n" unless $md5_test ne $md5_chek;

sub my_md5 {
   ...blah blah blah...
}