DB<4> sub tst (\[$@]) { print ref $_[0] }
  
  DB<5> $a="a";@a=(1..3)
  
  DB<6> tst $a
SCALAR
  DB<7> tst @a
ARRAY
  DB<8> tst "a"   # <--------- that's the point !
Type of arg 1 to main::tst must be one of [$@] (not constant item) at (eval 12)[/usr/share/perl/5.10/perl5db.pl:638] line 2, at EOF