I don't understand what you're trying to do. Note that according to perlrun, the -a autosplit switch can only be used with the -n or -p switches. (Update: The fact that you have what looks like a file name at the end of the OPed pseudo-code suggests you intended to use a -n or -p switch.)

You seem to be trying to use the ! $seen{ $string }++ idiom. Maybe the following will help clarify the situation although it doesn't actually use the autosplit feature (but neither does the OPed (pseudo)code).

Win8 Strawberry 5.30.3.1 (64)  Sun 07/17/2022 22:13:06
C:\@Work\Perl\monks\abdan
>cat story.rst
seen first
the first found
found second
seen second
first occurred
occurred second

>perl -ane "print if /(seen|found|occurred)/ && !$seen{ $1 }++" story.
+rst
seen first
the first found
first occurred
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