modified: my fault, I do this on one line, and I rewrote code when I post them, I trust they are same.
apparently, they are not.

and I find I should just quote this:"Called in list context, ...", from perldoc...

Only focus on op's condition...

use strict;
use warnings;
use feature 'say';
sub f {a => 1, b=>2}
my $foo = f();
my $bar = join '_', f();
say $foo;
say $bar;
#outputs
2
a_1_b_2
[download]

so that's why I assume the first argument keys accept is 'a', like join.
I just want to explain keys don't work for that single scalar value.

but but... keys looks like do not accept arguments in scalar context
if we write:

sub f {a => 1, b=>2}
my @all_key = keys f();
[download]

the warning message is "Type of arg 1 to keys must be hash or array (not subroutine entry)"
so I tried it:

sub f {a => 1, b=>2}
my %h = (c => 3);
say keys %h, f();
#output:
c
[download]

and then if we write:

sub f {a => 1, b=>2}
my %h = (c => 3);
say keys f(), %h;
#warnings:
Experimental keys on scalar is now forbidden at perl.pl line 11.
Type of arg 1 to keys must be hash or array (not subroutine entry) at 
+perl.pl line 11, near "),"
Execution of perl.pl aborted due to compilation errors.
[download]

so, this is why I think it is 'a'.
I guess the first argument of keys is the first element in list.
p.s. I should come back earlier...

edit: my fault, I type '}' as ')' ... thanks The Perlman and LanX, I had modified them:(
yes, I find every result different now. I write them on one line... here I redo all:

use strict;
use warnings;
use feature 'say';
sub f {a => 1, b=>2}
my $foo = f();
my $bar = join '_', f();
say $foo;
say $bar;
#output
2
a_1_b_2

sub f {a => 1, b=>2}
my @all_key = keys f();
#warnings:
Experimental keys on scalar is now forbidden at call_art.pl line 7.
Type of arg 1 to keys must be hash or array (not subroutine entry) at 
+call_art.pl line 7, near ");"
Execution of call_art.pl aborted due to compilation errors.

sub f {a => 1, b=>2}
my %h = (c => 3);
say keys %h, f();
#output
ca1b2

sub f {a => 1, b=>2}
my %h = (c => 3);
say keys f(), %h;
#warnings:
Experimental keys on scalar is now forbidden at call_art.pl line 8.
Type of arg 1 to keys must be hash or array (not subroutine entry) at 
+call_art.pl line 8, near "),"
Execution of call_art.pl aborted due to compilation errors.
[download]

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