Hello $h4X4_|=73}{,

To expand a little on hippo’s answer: If you print out the contents of @Foo::THING immediately after the statement beginning Foo::check_THING(1), you’ll see an additional, empty, anonymous hash has been autovivified by the action of referring to $THING[1]{check}¹:

 0:20 >perl 1665_SoPW.pl
Some Thing
No Thing
[{ check => 1 }, {}]
No Thing
Some Thing

 0:20 >
[download]

(I’m using Data::Dump.) And if you add this line:

no autovivification;
[download]

immediately before the line our @THING = ();, you’ll get the behaviour you expect:

 0:20 >perl 1665_SoPW.pl
Some Thing
No Thing
[{ check => 1 }]
Some Thing
No Thing

 0:24 >
[download]

(See the autovivification pragma.)

¹Note that you can replace check with any key name here and get the same result.

Hope that helps,