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: $?";

##</code><code>##

>c:\progs\perl5100\bin\perl t.pl
"-leprint for @ARGV" -- -p blastn -d "value of $hu_seq value of $hd_seq" -i "value of $contig" -o "value of $alignment"

>c:\progs\perl589\bin\perl t.pl
"-leprint for @ARGV" -- -p blastn -d "value of $hu_seq value of $hd_seq" -i "value of $contig" -o "value of $alignment"

>c:\progs\perl580\bin\perl t.pl
"-leprint for @ARGV" -- -p blastn -d "value of $hu_seq value of $hd_seq" -i "value of $contig" -o "value of $alignment"

>c:\progs\perl561\bin\perl t.pl
"-leprint for @ARGV" -- -p blastn -d "value of $hu_seq value of $hd_seq" -i "value of $contig" -o "value of $alignment"

##</code><code>##

>c:\progs\perl560\bin\perl t.pl
-leprint for @ARGV -- -p blastn -d value of $hu_seq value of $hd_seq -i value of $contig -o value of $alignment