Hi,

I'm not sure whether your problem is solved, the right hint was given by choroba. With the code line

open ( FH0, '<./$file' ) or die "Unable to open $file: $!\n";
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you try to open a file with the name './$file' and not with the filename './linux_complete_command_set.txt' as you like. The single quotes suppress variable expansion.

Another reason to use the three parameters form of open

open (FH0, '<', $file) or die ...
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Best regards
McA