You probably don't really mean ASCII when you say ASCII, but rather "control characters", or some such.

As for replacing certain characters, or ranges of characters, see tr, or s  (update: here are maybe more useful links, as the rather large section "Quote and Quote-like Operators" that the respective entries in perlfunc refer you to, might be somewhat distracting for the uninitiated: tr, s).

For detecting them, maybe something like this  (the set [^\x20-\x7e] denotes characters not in the range hex 20-7e (decimal 32-126) ):

my $s = "foo \x03 bar \x05 baz";

printf "detected strange char: 0x%x\n", ord  for $s =~ /[^\x20-\x7e]/g
+;

__END__
detected strange char: 0x3
detected strange char: 0x5
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