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use strict;
use warnings;
...
#outputs
2
a_1_b_2

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

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

sub f {a => 1, b=>2}
my %h = (c => 3);
...
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.

use strict;
use 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.