$ perl -E 'my $x = "A(B"; $x =~ s/\(/</; say $x'
A<B

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$ perl -E 'my $x = "A(B"; $x =~ y/(/</; say $x'
A<B

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$ perl -E 'my $x = "A(B"; $x = join "<", split /\(/, $x; say $x'
A<B

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$ perl -E 'my $x = "A(B"; substr $x, index($x, "("), 1, "<"; say $x'
A<B