use warnings;
use strict;

sub dive {
        my ($r, @p) = @_;
        return unless @p;
        unshift @p, 'default' if @p==1;
        $r = ref $r eq 'HASH' && exists $$r{$_} ? $$r{$_} : return for @p;
        return $r
}

my %var = (
        default => { foo => 'Default-Foo' },
        foo => { bar => 'Foo-Bar' },
        a => { deeply => { nested => { value => 'A-Deeply-Nested-Value' } } },
);

my $work = <<'EOF';
Hello, %foo
Blah, %foo.bar
Deep: %a.deeply.nested.value
EOF

print $work =~ s{ % ( \w+ (?:\.\w+)* ) }{ dive \%var, split /\./, $1 }xegr;

__END__

Hello, Default-Foo
Blah, Foo-Bar
Deep: A-Deeply-Nested-Value