I can confirm that your question is fuzzy and hard to decipher.

Code please, at best an SSCCE

see also I know what I mean. Why don't you? from brother father Grand

update

probably you mean the difference between

• my $str = 'a.*u'; m/$str/i

and

• my $pc = qr/a.*u/; m/$pc/i

?

see qr for more