Indirect Filehandles are discussed in perldoc perlopentut. The documentation for open may also be useful.

So your code should look like:

use strict;

open(my $foo, "<data.txt");     
while(<$foo>){                   
   chomp;
   print "Saw $_ in data.txt\n";
}
close $foo;
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For hints on lexical variables, you might try "Coping with scoping"