While all of the above answers are correct, I think they may be slightly missing the point of your question. What I understand you're asking is: "Why do the parens in $str do not screw up

(my $name = $str) =~ s{.*/}{};      # basename
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but do screw up

(my $path = $str) =~ s{$name}{};
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?"

And the answer to that is: they don't. It's not the parens in $str that are the problem in the second substitution, it's the parens in $name