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

# rest of the code as before..
print "\tclearing with undef \@arr:\n"     and   undef @arr;
...
Triggered:  Arraytrigger=ARRAY(0x6ab224) 0 a
        clearing with @arr=():
Triggered:  Arraytrigger=ARRAY(0x6ab224)

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

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