As using the string form of eval with untrusted input is a considerable security risk, and your markdown can easily contain double-quotes which break your code, maybe you are interested in using a templating system or simply applying this very small, very simple templating system? It allows you to restrict what variables get interpolated, and it prevents execution of arbitrary code:

sub interpolate {
    my ($text, %vars) = @_;
    $text =~ s!\$(\w+)|\$\{(\w+)\}! 
                 my $name = $1 || $2;
                 exists $vars{ $name } ? $vars{ $name } : '$' . $name
              !ge;
    return $text
};

print interpolate(
    'Hello $user, this is $var1. We also use the unknown variable $foo
+.',
    var1 => 'The value of var1',
    user => 'Je55eah',
    # ...
);
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