the filename is not passed in @ARGV

Of course there's nothing to stop you putting it there :)

my @args = @ARGV;
@ARGV = 'filename.txt';
my @found = grep { /^args[0]/ } <>;
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<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg