A technique similar to your hash suggestion can make the eval approach secure from code-insertion mischief (I'm guessing this is what you meant by "prone to error"):

my $string = 'foo: $foo and bar: $bar\n';
print $string, "\n";

my $foo = "foozleberry pie";
my $bar = "barzleberry foo";

$string =~ s/(\$\w+)/$1/gee;
print $string, "\n";
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Update: A hash is still a better choice because it automatically restricts which variables will be substituted to a set that you specify, which is almost surely desirable.