I have never used IO::Handle (without the "s"), but shift will remove the 1st argument on the command line for you:

my $filename = shift;
open(PAGE, $filename) || die "I can't open yourfile.txt";
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$filename would become "anyfile.txt" in your example command line.

Update: There are other ways of modifying a file in-place. A Super Search where title contains all of "edit", "file", "in", "place" show some ?node_id=3989;HIT=edit%20file%20in%20place;re=N

If your input file is large, these other methods would save memory by not storing the entire contents at once.