A few other things:

When you read the filename from STDIN it will include a trailing new-line character, so get ride of it with : chomp $filename;

When reporting an error from an open (or any other system related function) include the system error held in $!, so you know why it failed:

open (READFILE, $filename)|| die "Failed to opne $filename: $!";
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There is plenty more to do, but I suggest you get some basic code working,then embelish it.