You'll find a lot of places in Perl where they decided that a code expression that doesn't do anything useful should get a special case so that it does something useful.

> A no-op?

Please note that in mathematics an empty set is always a sub set of all other sets.

And so does the empty pattern in Perl always match.

If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match).

Hence not a no-op!°

> that a code expression that doesn't do anything useful should get a special case so that it does something useful.

I disagree. The "repeat the last match" feature should be dependent on an explicit modifier like eg s///b ²

I'd welcome a feature to switch off the current default behavior in favor of an explicit modifier.

Reasoning: The ratio of useful vs confusing is too bad.

Update

°) Demo:

~ $ perl -de0

...

  DB<1> $_='XXXX'

  DB<2> s/(?:)/Y/ # genuine empty pattern

  DB<3> p
YXXXX
  DB<3> s/(?:)/Z/g

  DB<4> p
ZYZXZXZXZXZ
  DB<4>
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²) in an earlier version I suggested s///l but this modifier is already taken. "b" (for before) seems to be free.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}

100% agree. At $job - 2, we spent several days tracking a bug caused by s/$regex/$replace/ where $regex got accidentally empty and the substitution started replacing unrelated parts of the data.

Besides /b or a feature, another way would be to introduce a new construct, e.g. (*LAST), which you can use similarly to (*SKIP) etc.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

OP's example of s/// would look like a no-op because the replacement is also empty.

Other than that, I agree.

I expected it just do nothing, But it's more useful to take last matched pattern.

s/(?:)// will do nothing (or rather, replace one-zero length string with another zero-length string). So, if you have a possibly-empty pattern in a variable, you could use

s/(?:$pat)//
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or

my $re = qr/$pat/;

s/$re//
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The latter was buggy before 5.18 as it resulted in the same as s/// when $pat contained an empty string.