Ah - \*DATA or \*IN - err, what does '\*' imply?

*NAME refers to the symbol table entry for "NAME". It contains $NAME (*NAME{SCALAR}), @NAME (*NAME{ARRAY}), %HASH (*NAME{HASH}), &NAME (*NAME{CODE}) and a number of other things, including a file handle (*NAME{IO}) and a directory handle.

So when you are doing

print FH ...
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It's effectively a shorthand for *FH, which is effectively a shorthand for *FH{IO}.

Things commonly accepted as file handles:

• IO object (*FH{IO}).
• reference to glob (\*FH).
• glob (*FH).
• name of the glob ("FH"). This is less supported than the others.

open( my $fh, ... ) assigns a reference to a glob to $fh (like \*FH).