Probably the easiest way is to give Perl an argument, like this:

cat file | perl -n0e 'BEGIN{$var = shift} #more code' myvar
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Then $var will contain the value 'myvar'. You might also use an environment variable accessible via the %ENV hash, like this:

VAR=myvar; export VAR; cat file | perl -n0e 'BEGIN{$var = $ENV{VAR}} #
+more code'
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I hope this is a useful answer to your question.