The question is how to get the $var variable into the perl code since I apparently can not get in is as regular argument while using the -e parameter.

Of course you can. There is no need for @ARGV to only contain a files list.

perl -le 'print shift' foo
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If the variable contains spaces, you may have to quote it.

You can even combine this with a file list passed as parameters:

perl -e '$x = shift; while (<>) { print "$x: $_" }' $2 $1
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or even

perl -nle 'BEGIN { $x = shift } print "$x: $_"' $2 $1
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