The =~ operator and s/// don't work the way you use them. You should also be seeing some warnings from Perl about

Useless use of private variable in void context at ... line 10.
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which should have made you suspicious about your code.

I think what you likely want is to learn about how =~ applies to variables, via perlop and perlre. I guess the following line could meet your intentions:

    (my $linefinal = $line) =~ s/.../.../;
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I see that you took some time to actually gather test input and output, but please test your posted code before posting it - your code doesn't read from the DATA handle.