my $string = 'aaaaa';
my $var = 'a';
my %var = (5 => 'hash interpolated');

print "String: $var{5}\n";
print "Regex: ", ($string =~ /$var{5}/ ? 'scalar interpolated' : 'hash interpolated'), "\n";
print "Regex (force hash): ", ($string =~ /${var{5}}/ ? 'scalar' : 'hash'), "\n";
print "Regex (force scalar): ", ($string =~ /${var}{5}/ ? 'scalar' : 'hash'), "\n";

# Output is:
# String: hash interpolated
# Regex: scalar interpolated
# Regex (force hash): hash
# Regex (force scalar): scalar