Can someone tell me where else this function chaining EXPR (chr hex) can be used in perl?

It's just a combination of function calls and map:

$ perl -MO=Deparse,-p -e 'map chr hex, @code_points'
map(chr(hex($_)), @code_points);
[download]

The behavior of chr and hex can be achieved with the (_) prototype, though prototypes are one of those "only use this if you know what you are doing" kind of thing because they change how Perl code is actually parsed (see also).

Parentheses can be omitted in calls to subs which the compiler has already seen.

$ perl -wMstrict -le 'sub foo {} foo "bar"'    # ok
$ perl -wMstrict -le 'sub foo {} foo("bar")'   # ok
$ perl -wMstrict -le 'foo("bar"); sub foo {}'  # ok
$ perl -wMstrict -le 'foo "bar"; sub foo {}'   # fails
String found where operator expected at -e line 1, near "foo "bar""
    (Do you need to predeclare foo?)
syntax error at -e line 1, near "foo "bar""
Execution of -e aborted due to compilation errors.
[download]

The only thing special here is map and grep, which have as an alternative to their map {...} ... and grep {...} ... forms the map ..., ... and grep ..., ... forms. The former two can be emulated using prototypes (e.g. (&@)), while the latter two can't.

Parentheses can be omitted in calls to subs which the compiler has already seen.

And to named operators such as hex and chr.

Seeking work! You can reach me at ikegami@adaelis.com

G'day pme,

Thankyou for your excellent question. Those of us who've been coding Perl for a quarter of a century or more, may often rely upon muscle memory rather than using any actual brain cells. When I start a new script, I often find that I've witten

#!/usr/bin/env perl

use strict;
use warnings;
[download]

before I've even thought about the problem.

Any Perl function that has function LIST can be expanded using function function-returning-list LIST. This is pretty much what 🦛 says.

Many commands allow the use of $_, check the documentation.

Hopefully, combined with other responses, that answers your question. Of course, if anything is still unclear, ask for further clarification.

Thank you all for your answers. Meanwhile I found answer myself to my question.

@a2 = map chr hex, @a1;
[download]

can be written as

@a2 = map chr(hex()), @a1;
# or
@a2 = map chr(hex($_)), @a1;
[download]

And the original syntax can be used in a simple assignment statement like this.

$x = char hex 41; # which means $x = 'A';
[download]

Any function (or sub) which returns a value (scalar, list, ref, whatever) can be acted upon by another function.