How does Perl parse unquoted bare words? (barewords, identifiers)

Unquoted words seem to have a great many meanings in Perl.

print STDERR $msg;

$hash{key}

func( param => $arg )

my $x = str;

How does one determine the meaning of these?

Solution

The following chart shows how Perl resolves identifiers in order of descending priority.

It also applies to identifiers chained by :: (or by ' before 5.42) unless otherwise stated. I'll call these "qualified identifiers".

Syntactically-defined meaning, when syntactically expected.

 sub foo { }          # «foo» («sub» is covered later)
 sub main::foo { }    # «main::foo» («sub» is covered later)
 method Class         # «Class» («method» is covered later)
 method Some::Class   # «Some::Class» («method» is covered later)
 $foo
 $main::foo
 //i
 =head
 <<FOO
 Class::
 Some::Class::
 LABEL:

String literal, when followed by a => or when the entirety of a hash index expression.

This doesn't apply to qualified identifiers.
```
 my %h = ( a => 1 );
 $h{a}
```
Variable name, when the entirety of the dereference expression.
```
 ${foo}
 ${main::foo}
```
Note that using the name of a keyword, named operator or declared sub will result in an ambiguous use warning.
Keyword.
```
 while (1) { }
 sub { }
 use
 __END__
```
Sub call, when the name of a previously imported sub.
```
 use Time::HiRes qw( time );
 time
 main::time
```
Invocation of a named list operator, named unary operator or named nullary operator.
```
 print $x, $y, $z;
 $c = chr $i;
 $t = time;
 $t = CORE::time;
```
Label, when used as the operand for next, last, redo or goto.

A qualified identifier treated as a label results in a compilation error since labels can't be qualified identifiers.
```
 next LABEL;
```

Sub call or inlined constant, when the name of a previously declared sub or constant.

 sub foo { }
 foo                          # Calls sub «foo»
 main::foo                    # Calls sub «foo»

 sub bar;
 bar                          # Calls sub «bar»

 use constant FOO => 123;
 FOO                          # Replaced with the value of the constant.

Indirect method call, when followed by a possibly-qualified identifier, a possibly-qualified identifier suffixed with ::, a scalar (incl array element or hash element) or a block.

 method Class           # Calls method «method» («Class» is covered earlier)
 method Some::Class     # Calls method «method» («Some::Class» is covered earlier)
 method Class::         # Calls method «method» («Class» is covered earlier)
 method Some::Class::   # Calls method «method» («Some::Class» is covered earlier)
 method $o              # Calls method «method»
 method { $o }          # Calls method «method»

 Base::method Class     # Calls method «Base::method» («Class» is covered earlier)

You can use the no indirect pragma to warn when code is parsed this way.

Glob, when used as the operand for an operator expecting a file handle.

 open(FH, '>', $qfn) or die $!;      # Equivalent to open(*FH, ...) or ...;
 print FH "Hello, World!\n";         # Equivalent to print *FH ...;
 print main::FH "Hello, World!\n";   # Equivalent to print *main::FH ...;

String literal, in the following situations:

When used as the invocant of a direct method call.

  Class->method(@args)         # Uses the string «Class» as the invocant.
  Some::Class->method(@args)   # Uses the string «Some::Class» as the invocant.

When used as the operand for unary minus.
```
  -foo
  -foo::bar
```

When used as an argument for the a sub parameter with a prototype of *.

  sub myprint(*@);
  myprint(FH, "Hello, World\n");
  myprint(main::FH, "Hello, World\n");

String literal. This is disallowed by use strict qw( subs );.

Hopefully, I didn't miss any.

Thanks to @mosvy, @Grinnz and @stevesliva! Each has uncovered a few cases I had missed.

CURRENTLY MISSING:

SUBNAME in sort SUBNAME.
BEGIN and similar. They sometimes act as keyword, and sometimes as a declared sub.

Importing a sub named print doesn't follow the above steps.

$ perl -M5.010 -e'
   use subs qw( time );
   eval { time; };
   say $@ =~ /Undefined sub/ ? "ok" : "bad";
'
ok

$ perl -M5.010 -e'
   use subs qw( system );
   eval { system; };
   say $@ =~ /Undefined sub/ ? "ok" : "bad";
'
ok

$ perl -M5.010 -e'
   use subs qw( print );
   eval { print; };
   say $@ =~ /Undefined sub/ ? "ok" : "bad";
'
bad

I don't know what makes that one special, and I don't know if there are others or not. I was guessing it's because print doesn't have a prototype, but system has no prototype either.