#! perl -slw
use strict;
use constant { X=>0, Y=>1, Z=>2, W=>3 };

my @first = (  -3.901,  ,9.352, ,-1.557 );
my @last  = ( -50.317,  -4.262, -17.720 );

my @v = (
    $last[X] - $first[X],
    $last[Y] - $first[Y],
    $last[Z] - $first[Z],
);

my $l = sqrt( $v[X]**2 + $v[Y]**2 + $v[Z]**2 );

my @unitVec = map $_ / $l, @v;

print "@unitVec";

__END__
[20:14:31.75] C:\test>unitVec.pl
-0.910112639703537 -0.266939707793087 -0.316919824963984