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

my %test_hash = ("foo_bar" => "baz");

my $test = "foo";

my $key = "${test}_bar";

my $var1 = "aaa/$test_hash{$key}";

printf("var1 = $var1\n");

my $var2 = "aaa/$test_hash{${test}_bar}";

printf("var2 = $var1\n");

##</code><code>##

> ./braces_inside_braces.pl 
Bareword found where operator expected at ./braces_inside_braces.pl line 14, near "${test}_bar"
        (Missing operator before _bar?)
syntax error at ./braces_inside_braces.pl line 14, near "${test}_bar"
Execution of ./braces_inside_braces.pl aborted due to compilation errors.