my $hu_seq    = 'value of $hu_seq';
my $hd_seq    = 'value of $hd_seq';
my $contig    = 'value of $contig';
my $alignment = 'value of $alignment';

my @command = (
   'echo',
   '-leprint for @ARGV', '--',
   '-p', 'blastn',
   '-d', "$hu_seq $hd_seq",
   '-i', $contig,
   '-o', $alignment
);

0 == system @command
   or die "system @command failed: $?";