Presuming any non-word character should be replaced with an underscore:

my $s = 'Hello ( World ).';
$s =~ s/\W+/_/g;  # Replace all sequences of [^a-zA-Z0-9_] with _
$s =~ s/_$//;     # Remove trailing _ if present
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Note that if you wanted to replace strings containing multiple sequential _, you'd have to change \W to the appropriate character class.