sub trigger{print "Triggered: ",map {defined $_ ? " $_" : 'UNDEF'}@_,"\n"}

##</code><code>##

# rest of the code as before..
print "\tclearing with undef \@arr:\n"     and   undef @arr;
print "\tSetting list:\n" and @arr = qw(a);
print "\tclearing with \@arr=undef:\n"     and   @arr=undef;
print "\tSetting list:\n" and @arr = qw(a);
print "\tclearing with \@arr=():\n"     and   @arr=();

__OUT__
        clearing with undef @arr:
Triggered:  Arraytrigger=ARRAY(0x6ab224)
        Setting list:
Triggered:  Arraytrigger=ARRAY(0x6ab224)
Triggered:  Arraytrigger=ARRAY(0x6ab224) 0 a
        clearing with @arr=undef:
Triggered:  Arraytrigger=ARRAY(0x6ab224)
Triggered:  Arraytrigger=ARRAY(0x6ab224) 0UNDEF
        Setting list:
Triggered:  Arraytrigger=ARRAY(0x6ab224)
Triggered:  Arraytrigger=ARRAY(0x6ab224) 0 a
        clearing with @arr=():
Triggered:  Arraytrigger=ARRAY(0x6ab224)

##</code><code>##

my $ref = \$arr[0];
print "\tsetting by reference:\n"     and   $ref=11;

__OUT__
        setting by reference:



##</code><code>##

print "\@arr [@arr]\n";