Because in perl's regular expression language parens have meaning, they're meta characters, specifically, they're called capturing parens. Anything matched in capturing parens, is stored in a number variable ($1,$2 ...)

see perlre, perlretut

To see how the regex engine does matching,  use re 'debug';

In reply to Re: Parens mess up regex substitute by Anonymous Monk
in thread Parens mess up regex substitute by BenHopkins

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