Perl CORE::say vs -E

Question

In this answer is used the perl one-liner as: ``` perl -we '... CORE::say "x=$x"' ``` What is the advantage using the `-e` and `CORE::say` instead of the shorter: `-E` and plain `say`, e.g.: ``` perl -wE '... say "x=$x"' ```

Accepted Answer

feature.pm was introduced to allow backwards-incompatible features to be added to Perl. `-E` enables all backward-incompatible features, which means a program that uses `-E` may break if you upgrade `perl`.

```
perl               -E'... say "foo";       ...'   # Forward-incompatible (5.10+)
perl -Mfeature=say -e'... say "foo";       ...'   # ok (5.10+)
perl -Mv5.10       -e'... say "foo";       ...'   # ok (5.10+)
perl -M5.010       -e'... say "foo";       ...'   # ok (5.10+)
perl               -e'... CORE::say "foo"; ...'   # ok (5.16+)
```

For example, say you wrote the following program in 2010:

```
perl -E'sub fc { my $acc=1; $acc*=$_ for 2..$_[0]; $acc } say fc(5);'
```

With the latest Perl in 2010 (5.12), the program outputs the following:

```
120
```

With the latest Perl in 2016 (5.24), the program outputs the following:

```
5
```

The difference is due to the addition of a feature to 5.16 that changes the meaning of that program when enabled. If one had avoided the use of `-E`, the program's behaviour would not have changed. Specifically, the following outputs `120` in 5.24:

```
perl -e'sub fc { my $acc=1; $acc*=$_ for 2..$_[0]; $acc } CORE::say fc(5);'
```

Perl CORE::say vs -E

Solution

Problem