Consider this snippet:

$bar = 'snafu';
$foo = '$bar';

$_ = '$bar snafu';

s/$foo/XYZ/;

print;
[download]

I think that you are expecting one of two results from this code, but I'm not sure which...

• $bar XYZ, as $foo interpolates to $bar, then $bar interpolates to snafu.
• XYZ snafu, as $foo interpolates to $bar, and $bar matches literally.

Neither of those would be correct behavior.

• Double-quoted strings don't interpolated recursively; neither do regexes. $bar = 'snafu'; $foo = '$bar'; $_ = "$foo"; leaves $_ with the value '$bar', not 'snafu'.
• If the value of $foo were '.*', would that match any number of characters, or only the two literal characters period and asterisk? It's the former, of course, because . and * are metacharacters. $ is the same way; backslash it if you want to match a literal dollar sign.

The correct output is $bar snafu, because the substitution doesn't find a match for m'$bar'