When you want to replace just characters, use the tr operator (simpler and faster than s///):

my $str = "(foo + bar)\n(baz * quux)\n";

# opening and closing parens and space => underscore
$str =~ tr/() /_/;

# newline => space
$str =~ tr/\n/ /;

print "<<$str>>\n";
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Output:

<<_foo_+_bar_ _baz_*_quux_ >>
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