Solution: Between brackets

Here is a possible solution to the task.

Code

say '$50' ~~ / <?after '$'> \d+ >> <!before '.'> /;

🦋 You can find the source code in the file percent-number.raku.

｢50｣

<?after '$'> is a positive lookbehind — it requires a $ just before the number — and <!before '.'> is a negative lookahead — it requires that a . does not follow. Neither the $ nor the surroundings become part of the match, so the result is just 50.
The >> is a word boundary marking the end of the number. Without it, greedy \d+ could backtrack: on $39.99 it would give up the 9 and match 3 (which is not followed by .). >> forces the whole number to be taken, so $3.99 and $39.99 correctly match nothing.
This is a case where a lookahead earns its keep: you cannot express “not followed by a decimal point” with an ordinary capturing group, because there is nothing there to capture — only a condition to check.