Are you, by any chance, using a lexical ('my') variable for $foo?  Symbolic references only work with package variables:

#!/usr/bin/perl

use strict;
use warnings;

our $foo="foo";
my $bar="f";

no strict 'refs';
print ${$bar.'oo'}, "\n";
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(BTW, just in case you're intending to create enumerated variables like $var1, $var2, etc., consider using an array instead...)