" ... what i tried doing is that some $ before num and some $ after it "

Do you, by any chance, mean you tried to deal with data that included a "$" sign... such as:  abc12$ or  $98abc or  $$zyx5abc or something similar?

If so, you'll have to escape the "$" sign (with a preceding backslash, "\") in your regex.

perl -E "my $s='$abc123'; if ($s =~ /\$/ ) { say 'Found a dollar-sign';}

which outputs:

Found a dollar-sign

Otherwise, (slightly oversimplified), the regex engine uses the "$" sign is only to mark an EOL.

Updated by quoting a possibly mis-understood part of OP's latest question.