Did you actually run it before you told me it was wrong?

If the second value is null as in something like

__DATA__
name1   4.5
name3
name5   6.5
name4   -7.9
name6   3.2
[download]

The the entire pattern will not match since the second value is required according to the regex. Also, a simple test of adding a print statement during the match will educate you that the $1 and $2 are not carried over.
Run this to see...

my @order = qw(name1 name2 name3 name4 name5 name6);
map{/(\S+)\s+(-?[\d.]+)/;$a{$1}=$2?$2:'';print "$1 -- $2 \n";}<DATA>;
map{push(@result,defined $a{$_} ? $a{$_} : 999)}@order;

print join("\n", @result),"\n";

__DATA__
name1   4.5
name3
name5   6.5
name4   -7.9
name6   3.2
[download]

Notice the added print line will show you that $1 and $2 are empty, not the previous values.

So, to conclude -- actually running it would have shown you that there was no error.