Seems to be a bug, however, explicitly capturing works:

#!/usr/bin/perl -w
use strict;
while(<DATA>){
    my @w = /^(\w+)\s+(\w+)$/;
    print "<$w[0]><$w[1]>\n";
}
__DATA__
abc def
xyz 6-7
123 456
[download]

gives

<abc><def>
Use of uninitialized value in concatenation (.) or string at x line 5,
+ <DATA> line 2.
Use of uninitialized value in concatenation (.) or string at x line 5,
+ <DATA> line 2.
<><>
<123><456>
[download]

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