For a description of the do operator, see perlop.

In the light of your reply, I'd probably do something along the lines of the following (in a script)...

use warnings;
use strict;

my %vars;

while (<>) {
    local @_ = split;
    $vars{$_[0]} = $_[1];
}
[download]

which takes each line of each file given on the command line and updates the %vars hash, keyed on the first field & values of the 2nd field - as per postings here and elsewhere on this thread.