Looks like a perl bug to me.... If you flatten out the while() loop:

#!/usr/bin/perl -w
use strict;

$_ = "abc def\n";
/^(\w+)\s+(\w+)$/;
print "<$1><$2>\n";

$_ = "xyz 6-7\n";
/^(\w+)\s+(\w+)$/;
print "<$1><$2>\n";

$_ = "123 456\n";
/^(\w+)\s+(\w+)$/;
print "<$1><$2>\n";
[download]

The output is

<abc><def>
<abc><def>
<123><456>
[download]

Which is what the expected output of your snippet should be. FWIW, replacing the while(<DATA>) with a for(@data) type construct, also producted erroneous results.

Before using $1 et all, you really should check to be sure your match succeeded:

while (<DATA>) {
  if (/^(\w+)\s+(\w+)$/) {
    print "<$1><$2>\n";
  } else  {
    print "Line $. didn't match\n";
  }
}
[download]

-Blake