That's just a matter of personal preference, not of "correct"ness.

It is also a matter of the intended audience: The less Perl they know, the more verbose might have to write.

perlstyle collects some stilistic wisdom that's widely adopted in the Perl community, but YMMV.

Update: In this specific case I'd either leave out the $_ altogether, or use an explicitly named variable, like

print $file if -d $file;
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