From the pos documentation:

pos directly accesses the location used by the regexp engine to store the offset, so assigning to pos will change that offset, and so will also influence the \G zero-width assertion in regular expressions.

So if we know that we have fixed-width data and we only want to match "foo", "ding", or "widget" starting at the 10th character of the string, we can assign to pos($string) to set the "last matched position" to something other than 0, and then use the \G anchor (technically a zero-width assertion, like ^ and $) like so:

while(<DATA>)
{
  pos() = 10;
  print if /\G(?:foo|ding|widget)/o;
}

__DATA__
 3.14 PI  foo
 6.28 2PI ding
   42 LUE answer
    7 LUK superstition
87.32 UNK widget
[download]

This code prints lines 1, 2, and 5